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PERLGLOSSARY(1)        Perl Programmers Reference Guide        PERLGLOSSARY(1)

       perlglossary - Perl Glossary

       A glossary of terms (technical and otherwise) used in the Perl documentation.  Other useful sources include the
       Free On-Line Dictionary of Computing <>;, the Jargon File
       <>;, and Wikipedia <>;.


       accessor methods
           A "method" used to indirectly inspect or update an "object"'s state (its instance variables).

       actual arguments
           The scalar values that you supply to a "function" or "subroutine" when you call it.  For instance, when you
           call "power("puff")", the string "puff" is the actual argument.  See also "argument" and "formal argu-

       address operator
           Some languages work directly with the memory addresses of values, but this can be like playing with fire.
           Perl provides a set of asbestos gloves for handling all memory management.  The closest to an address oper-
           ator in Perl is the backslash operator, but it gives you a "hard reference", which is much safer than a
           memory address.

           A well-defined sequence of steps, clearly enough explained that even a computer could do them.

           A nickname for something, which behaves in all ways as though you'd used the original name instead of the
           nickname.  Temporary aliases are implicitly created in the loop variable for "foreach" loops, in the $_
           variable for map or grep operators, in $a and $b during sort's comparison function, and in each element of
           @_ for the "actual arguments" of a subroutine call.  Permanent aliases are explicitly created in packages
           by importing symbols or by assignment to typeglobs.  Lexically scoped aliases for package variables are
           explicitly created by the our declaration.

           A list of possible choices from which you may select only one, as in "Would you like door A, B, or C?"
           Alternatives in regular expressions are separated with a single vertical bar: "|".  Alternatives in normal
           Perl expressions are separated with a double vertical bar: "||".  Logical alternatives in "Boolean" expres-
           sions are separated with either "||" or "or".

           Used to describe a "referent" that is not directly accessible through a named "variable".  Such a referent
           must be indirectly accessible through at least one "hard reference".  When the last hard reference goes
           away, the anonymous referent is destroyed without pity.

           The kind of computer you're working on, where one "kind" of computer means all those computers sharing a
           compatible machine language.  Since Perl programs are (typically) simple text files, not executable images,
           a Perl program is much less sensitive to the architecture it's running on than programs in other languages,
           such as C, that are compiled into machine code.  See also "platform" and "operating system".

           A piece of data supplied to a program, "subroutine", "function", or "method" to tell it what it's supposed
           to do.  Also called a "parameter".

           The name of the array containing the "argument" "vector" from the command line.  If you use the empty "<>"
           operator, "ARGV" is the name of both the "filehandle" used to traverse the arguments and the "scalar" con-
           taining the name of the current input file.

       arithmetical operator
           A "symbol" such as "+" or "/" that tells Perl to do the arithmetic you were supposed to learn in grade

           An ordered sequence of values, stored such that you can easily access any of the values using an integer
           "subscript" that specifies the value's "offset" in the sequence.

       array context
           An archaic expression for what is more correctly referred to as "list context".

           The American Standard Code for Information Interchange (a 7-bit character set adequate only for poorly rep-
           resenting English text).  Often used loosely to describe the lowest 128 values of the various ISO-8859-X
           character sets, a bunch of mutually incompatible 8-bit codes best described as half ASCII.  See also "Uni-

           A component of a "regular expression" that must be true for the pattern to match but does not necessarily
           match any characters itself.  Often used specifically to mean a "zero width" assertion.

           An "operator" whose assigned mission in life is to change the value of a "variable".

       assignment operator
           Either a regular "assignment", or a compound "operator" composed of an ordinary assignment and some other
           operator, that changes the value of a variable in place, that is, relative to its old value.  For example,
           "$a += 2" adds 2 to $a.

       associative array
           See "hash".  Please.

           Determines whether you do the left "operator" first or the right "operator" first when you have "A "opera-
           tor" B "operator" C" and the two operators are of the same precedence.  Operators like "+" are left asso-
           ciative, while operators like "**" are right associative.  See perlop for a list of operators and their

           Said of events or activities whose relative temporal ordering is indeterminate because too many things are
           going on at once.  Hence, an asynchronous event is one you didn't know when to expect.

           A "regular expression" component potentially matching a "substring" containing one or more characters and
           treated as an indivisible syntactic unit by any following "quantifier".  (Contrast with an "assertion" that
           matches something of "zero width" and may not be quantified.)

       atomic operation
           When Democritus gave the word "atom" to the indivisible bits of matter, he meant literally something that
           could not be cut: a- (not) + tomos (cuttable).  An atomic operation is an action that can't be interrupted,
           not one forbidden in a nuclear-free zone.

           A new feature that allows the declaration of variables and subroutines with modifiers as in "sub foo :
           locked method".  Also, another name for an "instance variable" of an "object".

           A feature of "operator overloading" of objects, whereby the behavior of certain operators can be reasonably
           deduced using more fundamental operators.  This assumes that the overloaded operators will often have the
           same relationships as the regular operators.  See perlop.

           To add one to something automatically, hence the name of the "++" operator.  To instead subtract one from
           something automatically is known as an "autodecrement".

           To load on demand.  (Also called "lazy" loading.)  Specifically, to call an AUTOLOAD subroutine on behalf
           of an undefined subroutine.

           To split a string automatically, as the -a "switch" does when running under -p or -n in order to emulate
           "awk".  (See also the AutoSplit module, which has nothing to do with the -a switch, but a lot to do with

           A Greco-Roman word meaning "to bring oneself to life".  In Perl, storage locations (lvalues) spontaneously
           generate themselves as needed, including the creation of any "hard reference" values to point to the next
           level of storage.  The assignment "$a[5][5][5][5][5] = "quintet"" potentially creates five scalar storage
           locations, plus four references (in the first four scalar locations) pointing to four new anonymous arrays
           (to hold the last four scalar locations).  But the point of autovivification is that you don't have to
           worry about it.

       AV  Short for "array value", which refers to one of Perl's internal data types that holds an "array".  The "AV"
           type is a subclass of "SV".

       awk Descriptive editing term--short for "awkward".  Also coincidentally refers to a venerable text-processing
           language from which Perl derived some of its high-level ideas.


           A substring captured by a subpattern within unadorned parentheses in a "regex".  Backslashed decimal num-
           bers ("\1", "\2", etc.)  later in the same pattern refer back to the corresponding subpattern in the cur-
           rent match.  Outside the pattern, the numbered variables ($1, $2, etc.) continue to refer to these same
           values, as long as the pattern was the last successful match of the current dynamic scope.

           The practice of saying, "If I had to do it all over, I'd do it differently," and then actually going back
           and doing it all over differently.  Mathematically speaking, it's returning from an unsuccessful recursion
           on a tree of possibilities.  Perl backtracks when it attempts to match patterns with a "regular expres-
           sion", and its earlier attempts don't pan out.  See "Backtracking" in perlre.

       backward compatibility
           Means you can still run your old program because we didn't break any of the features or bugs it was relying

           A word sufficiently ambiguous to be deemed illegal under use strict 'subs'.  In the absence of that stric-
           ture, a bareword is treated as if quotes were around it.

       base class
           A generic "object" type; that is, a "class" from which other, more specific classes are derived genetically
           by "inheritance".  Also called a "superclass" by people who respect their ancestors.

           From Swift: someone who eats eggs big end first.  Also used of computers that store the most significant
           "byte" of a word at a lower byte address than the least significant byte.  Often considered superior to
           little-endian machines.  See also "little-endian".

           Having to do with numbers represented in base 2.  That means there's basically two numbers, 0 and 1.  Also
           used to describe a "non-text file", presumably because such a file makes full use of all the binary bits in
           its bytes.  With the advent of "Unicode", this distinction, already suspect, loses even more of its mean-

       binary operator
           An "operator" that takes two operands.

           To assign a specific "network address" to a "socket".

       bit An integer in the range from 0 to 1, inclusive.  The smallest possible unit of information storage.  An
           eighth of a "byte" or of a dollar.  (The term "Pieces of Eight" comes from being able to split the old
           Spanish dollar into 8 bits, each of which still counted for money.  That's why a 25-cent piece today is
           still "two bits".)

       bit shift
           The movement of bits left or right in a computer word, which has the effect of multiplying or dividing by a
           power of 2.

       bit string
           A sequence of bits that is actually being thought of as a sequence of bits, for once.

           In corporate life, to grant official approval to a thing, as in, "The VP of Engineering has blessed our
           WebCruncher project." Similarly in Perl, to grant official approval to a "referent" so that it can function
           as an "object", such as a WebCruncher object.  See "bless" in perlfunc.

           What a "process" does when it has to wait for something: "My process blocked waiting for the disk."  As an
           unrelated noun, it refers to a large chunk of data, of a size that the "operating system" likes to deal
           with (normally a power of two such as 512 or 8192).  Typically refers to a chunk of data that's coming from
           or going to a disk file.

           A syntactic construct consisting of a sequence of Perl statements that is delimited by braces.  The "if"
           and "while" statements are defined in terms of BLOCKs, for instance.  Sometimes we also say "block" to mean
           a lexical scope; that is, a sequence of statements that act like a "BLOCK", such as within an eval or a
           file, even though the statements aren't delimited by braces.

       block buffering
           A method of making input and output efficient by passing one "block" at a time.  By default, Perl does
           block buffering to disk files.  See "buffer" and "command buffering".

           A value that is either "true" or "false".

       Boolean context
           A special kind of "scalar context" used in conditionals to decide whether the "scalar value" returned by an
           expression is "true" or "false".  Does not evaluate as either a string or a number.  See "context".

           A spot in your program where you've told the debugger to stop execution so you can poke around and see
           whether anything is wrong yet.

           To send a "datagram" to multiple destinations simultaneously.

       BSD A psychoactive drug, popular in the 80s, probably developed at U. C. Berkeley or thereabouts.  Similar in
           many ways to the prescription-only medication called "System V", but infinitely more useful.  (Or, at
           least, more fun.)  The full chemical name is "Berkeley Standard Distribution".

           A location in a "hash table" containing (potentially) multiple entries whose keys "hash" to the same hash
           value according to its hash function.  (As internal policy, you don't have to worry about it, unless you're
           into internals, or policy.)

           A temporary holding location for data.  Block buffering means that the data is passed on to its destination
           whenever the buffer is full.  Line buffering means that it's passed on whenever a complete line is
           received.  Command buffering means that it's passed every time you do a print command (or equivalent).  If
           your output is unbuffered, the system processes it one byte at a time without the use of a holding area.
           This can be rather inefficient.

           A "function" that is predefined in the language.  Even when hidden by "overriding", you can always get at a
           built-in function by qualifying its name with the "CORE::" pseudo-package.

           A group of related modules on "CPAN".  (Also, sometimes refers to a group of command-line switches grouped
           into one "switch cluster".)

           A piece of data worth eight bits in most places.

           A pidgin-like language spoken among 'droids when they don't wish to reveal their orientation (see
           "endian").  Named after some similar languages spoken (for similar reasons) between compilers and inter-
           preters in the late 20th century.  These languages are characterized by representing everything as a non-
           architecture-dependent sequence of bytes.


       C   A language beloved by many for its inside-out "type" definitions, inscrutable "precedence" rules, and heavy
           "overloading" of the function-call mechanism.  (Well, actually, people first switched to C because they
           found lowercase identifiers easier to read than upper.)  Perl is written in C, so it's not surprising that
           Perl borrowed a few ideas from it.

       C preprocessor
           The typical C compiler's first pass, which processes lines beginning with "#" for conditional compilation
           and macro definition and does various manipulations of the program text based on the current definitions.
           Also known as cpp(1).

       call by reference
           An "argument"-passing mechanism in which the "formal arguments" refer directly to the "actual arguments",
           and the "subroutine" can change the actual arguments by changing the formal arguments.  That is, the formal
           argument is an "alias" for the actual argument.  See also "call by value".

       call by value
           An "argument"-passing mechanism in which the "formal arguments" refer to a copy of the "actual arguments",
           and the "subroutine" cannot change the actual arguments by changing the formal arguments.  See also "call
           by reference".

           A "handler" that you register with some other part of your program in the hope that the other part of your
           program will "trigger" your handler when some event of interest transpires.

           Reduced to a standard form to facilitate comparison.

           The use of parentheses around a "subpattern" in a "regular expression" to store the matched "substring" as
           a "backreference".  (Captured strings are also returned as a list in "list context".)

           A small integer representative of a unit of orthography.  Historically, characters were usually stored as
           fixed-width integers (typically in a byte, or maybe two, depending on the character set), but with the
           advent of UTF-8, characters are often stored in a variable number of bytes depending on the size of the
           integer that represents the character.  Perl manages this transparently for you, for the most part.

       character class
           A square-bracketed list of characters used in a "regular expression" to indicate that any character of the
           set may occur at a given point.  Loosely, any predefined set of characters so used.

       character property
           A predefined "character class" matchable by the "\p" "metasymbol".  Many standard properties are defined
           for "Unicode".

       circumfix operator
           An "operator" that surrounds its "operand", like the angle operator, or parentheses, or a hug.

           A user-defined "type", implemented in Perl via a "package" that provides (either directly or by inheri-
           tance) methods (that is, subroutines) to handle instances of the class (its objects).  See also "inheri-

       class method
           A "method" whose "invocant" is a "package" name, not an "object" reference.  A method associated with the
           class as a whole.

           In networking, a "process" that initiates contact with a "server" process in order to exchange data and
           perhaps receive a service.

           A "cluster" used to restrict the scope of a "regular expression modifier".

           An "anonymous" subroutine that, when a reference to it is generated at run time, keeps track of the identi-
           ties of externally visible lexical variables even after those lexical variables have supposedly gone out of
           "scope".  They're called "closures" because this sort of behavior gives mathematicians a sense of closure.

           A parenthesized "subpattern" used to group parts of a "regular expression" into a single "atom".

           The word returned by the ref function when you apply it to a reference to a subroutine.  See also "CV".

       code generator
           A system that writes code for you in a low-level language, such as code to implement the backend of a com-
           piler.  See "program generator".

       code subpattern
           A "regular expression" subpattern whose real purpose is to execute some Perl code, for example, the
           "(?{...})" and "(??{...})" subpatterns.

       collating sequence
           The order into which characters sort.  This is used by "string" comparison routines to decide, for example,
           where in this glossary to put "collating sequence".

           In "shell" programming, the syntactic combination of a program name and its arguments.  More loosely, any-
           thing you type to a shell (a command interpreter) that starts it doing something.  Even more loosely, a
           Perl "statement", which might start with a "label" and typically ends with a semicolon.

       command buffering
           A mechanism in Perl that lets you store up the output of each Perl "command" and then flush it out as a
           single request to the "operating system".  It's enabled by setting the $| ($AUTOFLUSH) variable to a true
           value.  It's used when you don't want data sitting around not going where it's supposed to, which may hap-
           pen because the default on a "file" or "pipe" is to use "block buffering".

       command name
           The name of the program currently executing, as typed on the command line.  In C, the "command" name is
           passed to the program as the first command-line argument.  In Perl, it comes in separately as $0.

       command-line arguments
           The values you supply along with a program name when you tell a "shell" to execute a "command".  These val-
           ues are passed to a Perl program through @ARGV.

           A remark that doesn't affect the meaning of the program.  In Perl, a comment is introduced by a "#" charac-
           ter and continues to the end of the line.

       compilation unit
           The "file" (or "string", in the case of eval) that is currently being compiled.

       compile phase
           Any time before Perl starts running your main program.  See also "run phase".  Compile phase is mostly
           spent in "compile time", but may also be spent in "run time" when "BEGIN" blocks, use declarations, or con-
           stant subexpressions are being evaluated.  The startup and import code of any use declaration is also run
           during compile phase.

       compile time
           The time when Perl is trying to make sense of your code, as opposed to when it thinks it knows what your
           code means and is merely trying to do what it thinks your code says to do, which is "run time".

           Strictly speaking, a program that munches up another program and spits out yet another file containing the
           program in a "more executable" form, typically containing native machine instructions.  The perl program is
           not a compiler by this definition, but it does contain a kind of compiler that takes a program and turns it
           into a more executable form (syntax trees) within the perl process itself, which the "interpreter" then
           interprets.  There are, however, extension modules to get Perl to act more like a "real" compiler.  See O.

           A "constructor" for a "referent" that isn't really an "object", like an anonymous array or a hash (or a
           sonata, for that matter).  For example, a pair of braces acts as a composer for a hash, and a pair of
           brackets acts as a composer for an array.  See "Making References" in perlref.

           The process of gluing one cat's nose to another cat's tail.  Also, a similar operation on two strings.

           Something "iffy".  See "Boolean context".

           In telephony, the temporary electrical circuit between the caller's and the callee's phone.  In networking,
           the same kind of temporary circuit between a "client" and a "server".

           As a noun, a piece of syntax made up of smaller pieces.  As a transitive verb, to create an "object" using
           a "constructor".

           Any "class method", instance "method", or "subroutine" that composes, initializes, blesses, and returns an
           "object".  Sometimes we use the term loosely to mean a "composer".

           The surroundings, or environment.  The context given by the surrounding code determines what kind of data a
           particular "expression" is expected to return.  The three primary contexts are "list context", "scalar con-
           text", and "void context".  Scalar context is sometimes subdivided into "Boolean context", "numeric con-
           text", "string context", and "void context".  There's also a "don't care" scalar context (which is dealt
           with in Programming Perl, Third Edition, Chapter 2, "Bits and Pieces" if you care).

           The treatment of more than one physical "line" as a single logical line.  "Makefile" lines are continued by
           putting a backslash before the "newline".  Mail headers as defined by RFC 822 are continued by putting a
           space or tab after the newline.  In general, lines in Perl do not need any form of continuation mark,
           because "whitespace" (including newlines) is gleefully ignored.  Usually.

       core dump
           The corpse of a "process", in the form of a file left in the "working directory" of the process, usually as
           a result of certain kinds of fatal error.

           The Comprehensive Perl Archive Network.  (See "What modules and extensions are available for Perl? What is
           CPAN? What does CPAN/src/... mean?" in perlfaq2).

           Someone who breaks security on computer systems.  A cracker may be a true "hacker" or only a "script kid-

       current package
           The "package" in which the current statement is compiled.  Scan backwards in the text of your program
           through the current lexical scope or any enclosing lexical scopes till you find a package declaration.
           That's your current package name.

       current working directory
           See "working directory".

       currently selected output channel
           The last "filehandle" that was designated with select("FILEHANDLE"); "STDOUT", if no filehandle has been

       CV  An internal "code value" typedef, holding a "subroutine".  The "CV" type is a subclass of "SV".


       dangling statement
           A bare, single "statement", without any braces, hanging off an "if" or "while" conditional.  C allows them.
           Perl doesn't.

       data structure
           How your various pieces of data relate to each other and what shape they make when you put them all
           together, as in a rectangular table or a triangular-shaped tree.

       data type
           A set of possible values, together with all the operations that know how to deal with those values.  For
           example, a numeric data type has a certain set of numbers that you can work with and various mathematical
           operations that you can do on the numbers but would make little sense on, say, a string such as "Kilroy".
           Strings have their own operations, such as "concatenation".  Compound types made of a number of smaller
           pieces generally have operations to compose and decompose them, and perhaps to rearrange them.  Objects
           that model things in the real world often have operations that correspond to real activities.  For
           instance, if you model an elevator, your elevator object might have an "open_door()" "method".

           A packet of data, such as a "UDP" message, that (from the viewpoint of the programs involved) can be sent
           independently over the network.  (In fact, all packets are sent independently at the "IP" level, but
           "stream" protocols such as "TCP" hide this from your program.)

       DBM Stands for "Data Base Management" routines, a set of routines that emulate an "associative array" using
           disk files.  The routines use a dynamic hashing scheme to locate any entry with only two disk accesses.
           DBM files allow a Perl program to keep a persistent "hash" across multiple invocations.  You can tie your
           hash variables to various DBM implementations--see AnyDBM_File and DB_File.

           An "assertion" that states something exists and perhaps describes what it's like, without giving any com-
           mitment as to how or where you'll use it.  A declaration is like the part of your recipe that says, "two
           cups flour, one large egg, four or five tadpoles..."  See "statement" for its opposite.  Note that some
           declarations also function as statements.  Subroutine declarations also act as definitions if a body is

           To subtract a value from a variable, as in "decrement $x" (meaning to remove 1 from its value) or "decre-
           ment $x by 3".

           A "value" chosen for you if you don't supply a value of your own.

           Having a meaning.  Perl thinks that some of the things people try to do are devoid of meaning, in particu-
           lar, making use of variables that have never been given a "value" and performing certain operations on data
           that isn't there.  For example, if you try to read data past the end of a file, Perl will hand you back an
           undefined value.  See also "false" and "defined" in perlfunc.

           A "character" or "string" that sets bounds to an arbitrarily-sized textual object, not to be confused with
           a "separator" or "terminator".  "To delimit" really just means "to surround" or "to enclose" (like these
           parentheses are doing).

           A fancy computer science term meaning "to follow a "reference" to what it points to".  The "de" part of it
           refers to the fact that you're taking away one level of "indirection".

       derived class
           A "class" that defines some of its methods in terms of a more generic class, called a "base class".  Note
           that classes aren't classified exclusively into base classes or derived classes: a class can function as
           both a derived class and a base class simultaneously, which is kind of classy.

           See "file descriptor".

           To deallocate the memory of a "referent" (first triggering its "DESTROY" method, if it has one).

           A special "method" that is called when an "object" is thinking about destroying itself.  A Perl program's
           "DESTROY" method doesn't do the actual destruction; Perl just triggers the method in case the "class" wants
           to do any associated cleanup.

           A whiz-bang hardware gizmo (like a disk or tape drive or a modem or a joystick or a mouse) attached to your
           computer, that the "operating system" tries to make look like a "file" (or a bunch of files).  Under Unix,
           these fake files tend to live in the /dev directory.

           A "pod" directive.  See perlpod.

           A special file that contains other files.  Some operating systems call these "folders", "drawers", or "cat-

       directory handle
           A name that represents a particular instance of opening a directory to read it, until you close it.  See
           the opendir function.

           To send something to its correct destination.  Often used metaphorically to indicate a transfer of program-
           matic control to a destination selected algorithmically, often by lookup in a table of function references
           or, in the case of object methods, by traversing the inheritance tree looking for the most specific defini-
           tion for the method.

           A standard, bundled release of a system of software.  The default usage implies source code is included.
           If that is not the case, it will be called a "binary-only" distribution.

           An enchantment, illusion, phantasm, or jugglery.  Said when Perl's magical "dwimmer" effects don't do what
           you expect, but rather seem to be the product of arcane dweomercraft, sorcery, or wonder working.  [From
           Old English]

           DWIM is an acronym for "Do What I Mean", the principle that something should just do what you want it to do
           without an undue amount of fuss.  A bit of code that does "dwimming" is a "dwimmer".  Dwimming can require
           a great deal of behind-the-scenes magic, which (if it doesn't stay properly behind the scenes) is called a
           "dweomer" instead.

       dynamic scoping
           Dynamic scoping works over a dynamic scope, making variables visible throughout the rest of the "block" in
           which they are first used and in any subroutines that are called by the rest of the block.  Dynamically
           scoped variables can have their values temporarily changed (and implicitly restored later) by a local oper-
           ator.  (Compare "lexical scoping".)  Used more loosely to mean how a subroutine that is in the middle of
           calling another subroutine "contains" that subroutine at "run time".


           Derived from many sources.  Some would say too many.

           A basic building block.  When you're talking about an "array", it's one of the items that make up the

           When something is contained in something else, particularly when that might be considered surprising: "I've
           embedded a complete Perl interpreter in my editor!"

       empty subclass test
           The notion that an empty "derived class" should behave exactly like its "base class".

       en passant
           When you change a "value" as it is being copied.  [From French, "in passing", as in the exotic pawn-captur-
           ing maneuver in chess.]

           The veil of abstraction separating the "interface" from the "implementation" (whether enforced or not),
           which mandates that all access to an "object"'s state be through methods alone.

           See "little-endian" and "big-endian".

           The collective set of environment variables your "process" inherits from its parent.  Accessed via %ENV.

       environment variable
           A mechanism by which some high-level agent such as a user can pass its preferences down to its future off-
           spring (child processes, grandchild processes, great-grandchild processes, and so on).  Each environment
           variable is a "key"/"value" pair, like one entry in a "hash".

       EOF End of File.  Sometimes used metaphorically as the terminating string of a "here document".

           The error number returned by a "syscall" when it fails.  Perl refers to the error by the name $! (or
           $OS_ERROR if you use the English module).

           See "exception" or "fatal error".

       escape sequence
           See "metasymbol".

           A fancy term for an error.  See "fatal error".

       exception handling
           The way a program responds to an error.  The exception handling mechanism in Perl is the eval operator.

           To throw away the current "process"'s program and replace it with another without exiting the process or
           relinquishing any resources held (apart from the old memory image).

       executable file
           A "file" that is specially marked to tell the "operating system" that it's okay to run this file as a pro-
           gram.  Usually shortened to "executable".

           To run a program or "subroutine".  (Has nothing to do with the kill built-in, unless you're trying to run a
           "signal handler".)

       execute bit
           The special mark that tells the operating system it can run this program.  There are actually three execute
           bits under Unix, and which bit gets used depends on whether you own the file singularly, collectively, or
           not at all.

       exit status
           See "status".

           To make symbols from a "module" available for "import" by other modules.

           Anything you can legally say in a spot where a "value" is required.  Typically composed of literals, vari-
           ables, operators, functions, and "subroutine" calls, not necessarily in that order.

           A Perl module that also pulls in compiled C or C++ code.  More generally, any experimental option that can
           be compiled into Perl, such as multithreading.


           In Perl, any value that would look like "" or "0" if evaluated in a string context.  Since undefined values
           evaluate to "", all undefined values are false, but not all false values are undefined.

       FAQ Frequently Asked Question (although not necessarily frequently answered, especially if the answer appears
           in the Perl FAQ shipped standard with Perl).

       fatal error
           An uncaught "exception", which causes termination of the "process" after printing a message on your "stan-
           dard error" stream.  Errors that happen inside an eval are not fatal.  Instead, the eval terminates after
           placing the exception message in the $@ ($EVAL_ERROR) variable.  You can try to provoke a fatal error with
           the die operator (known as throwing or raising an exception), but this may be caught by a dynamically
           enclosing eval.  If not caught, the die becomes a fatal error.

           A single piece of numeric or string data that is part of a longer "string", "record", or "line".  Variable-
           width fields are usually split up by separators (so use split to extract the fields), while fixed-width
           fields are usually at fixed positions (so use unpack).  Instance variables are also known as fields.

           First In, First Out.  See also "LIFO".  Also, a nickname for a "named pipe".

           A named collection of data, usually stored on disk in a "directory" in a "filesystem".  Roughly like a doc-
           ument, if you're into office metaphors.  In modern filesystems, you can actually give a file more than one
           name.  Some files have special properties, like directories and devices.

       file descriptor
           The little number the "operating system" uses to keep track of which opened "file" you're talking about.
           Perl hides the file descriptor inside a "standard I/O" stream and then attaches the stream to a "filehan-

       file test operator
           A built-in unary operator that you use to determine whether something is "true" about a file, such as "-o
           $filename" to test whether you're the owner of the file.

           A "wildcard" match on filenames.  See the glob function.

           An identifier (not necessarily related to the real name of a file) that represents a particular instance of
           opening a file until you close it.  If you're going to open and close several different files in succes-
           sion, it's fine to open each of them with the same filehandle, so you don't have to write out separate code
           to process each file.

           One name for a file.  This name is listed in a "directory", and you can use it in an open to tell the
           "operating system" exactly which file you want to open, and associate the file with a "filehandle" which
           will carry the subsequent identity of that file in your program, until you close it.

           A set of directories and files residing on a partition of the disk.  Sometimes known as a "partition".  You
           can change the file's name or even move a file around from directory to directory within a filesystem with-
           out actually moving the file itself, at least under Unix.

           A program designed to take a "stream" of input and transform it into a stream of output.

           We tend to avoid this term because it means so many things.  It may mean a command-line "switch" that takes
           no argument itself (such as Perl's -n and -p flags) or, less frequently, a single-bit indicator (such as
           the "O_CREAT" and "O_EXCL" flags used in sysopen).

       floating point
           A method of storing numbers in "scientific notation", such that the precision of the number is independent
           of its magnitude (the decimal point "floats").  Perl does its numeric work with floating-point numbers
           (sometimes called "floats"), when it can't get away with using integers.  Floating-point numbers are mere
           approximations of real numbers.

           The act of emptying a "buffer", often before it's full.

           Far More Than Everything You Ever Wanted To Know.  An exhaustive treatise on one narrow topic, something of
           a super-"FAQ".  See Tom for far more.

           To create a child "process" identical to the parent process at its moment of conception, at least until it
           gets ideas of its own.  A thread with protected memory.

       formal arguments
           The generic names by which a "subroutine" knows its arguments.  In many languages, formal arguments are
           always given individual names, but in Perl, the formal arguments are just the elements of an array.  The
           formal arguments to a Perl program are $ARGV[0], $ARGV[1], and so on.  Similarly, the formal arguments to a
           Perl subroutine are $_[0], $_[1], and so on.  You may give the arguments individual names by assigning the
           values to a my list.  See also "actual arguments".

           A specification of how many spaces and digits and things to put somewhere so that whatever you're printing
           comes out nice and pretty.

       freely available
           Means you don't have to pay money to get it, but the copyright on it may still belong to someone else (like

       freely redistributable
           Means you're not in legal trouble if you give a bootleg copy of it to your friends and we find out about
           it.  In fact, we'd rather you gave a copy to all your friends.

           Historically, any software that you give away, particularly if you make the source code available as well.
           Now often called "open source software".  Recently there has been a trend to use the term in contradistinc-
           tion to "open source software", to refer only to free software released under the Free Software Founda-
           tion's GPL (General Public License), but this is difficult to justify etymologically.

           Mathematically, a mapping of each of a set of input values to a particular output value.  In computers,
           refers to a "subroutine" or "operator" that returns a "value".  It may or may not have input values (called

       funny character
           Someone like Larry, or one of his peculiar friends.  Also refers to the strange prefixes that Perl requires
           as noun markers on its variables.

       garbage collection
           A misnamed feature--it should be called, "expecting your mother to pick up after you".  Strictly speaking,
           Perl doesn't do this, but it relies on a reference-counting mechanism to keep things tidy.  However, we
           rarely speak strictly and will often refer to the reference-counting scheme as a form of garbage collec-
           tion.  (If it's any comfort, when your interpreter exits, a "real" garbage collector runs to make sure
           everything is cleaned up if you've been messy with circular references and such.)


       GID Group ID--in Unix, the numeric group ID that the "operating system" uses to identify you and members of
           your "group".

           Strictly, the shell's "*" character, which will match a "glob" of characters when you're trying to generate
           a list of filenames.  Loosely, the act of using globs and similar symbols to do pattern matching.  See also
           "fileglob" and "typeglob".

           Something you can see from anywhere, usually used of variables and subroutines that are visible everywhere
           in your program.  In Perl, only certain special variables are truly global--most variables (and all subrou-
           tines) exist only in the current "package".  Global variables can be declared with our.  See "our" in perl-

       global destruction
           The "garbage collection" of globals (and the running of any associated object destructors) that takes place
           when a Perl "interpreter" is being shut down.  Global destruction should not be confused with the Apoca-
           lypse, except perhaps when it should.

       glue language
           A language such as Perl that is good at hooking things together that weren't intended to be hooked

           The size of the pieces you're dealing with, mentally speaking.

           A "subpattern" whose "quantifier" wants to match as many things as possible.

           Originally from the old Unix editor command for "Globally search for a Regular Expression and Print it",
           now used in the general sense of any kind of search, especially text searches.  Perl has a built-in grep
           function that searches a list for elements matching any given criterion, whereas the grep(1) program
           searches for lines matching a "regular expression" in one or more files.

           A set of users of which you are a member.  In some operating systems (like Unix), you can give certain file
           access permissions to other members of your group.

       GV  An internal "glob value" typedef, holding a "typeglob".  The "GV" type is a subclass of "SV".


           Someone who is brilliantly persistent in solving technical problems, whether these involve golfing, fight-
           ing orcs, or programming.  Hacker is a neutral term, morally speaking.  Good hackers are not to be confused
           with evil crackers or clueless script kiddies.  If you confuse them, we will presume that you are either
           evil or clueless.

           A "subroutine" or "method" that is called by Perl when your program needs to respond to some internal
           event, such as a "signal", or an encounter with an operator subject to "operator overloading".  See also

       hard reference
           A "scalar" "value" containing the actual address of a "referent", such that the referent's "reference"
           count accounts for it.  (Some hard references are held internally, such as the implicit reference from one
           of a "typeglob"'s variable slots to its corresponding referent.)  A hard reference is different from a
           "symbolic reference".

           An unordered association of "key"/"value" pairs, stored such that you can easily use a string "key" to look
           up its associated data "value".  This glossary is like a hash, where the word to be defined is the key, and
           the definition is the value.  A hash is also sometimes septisyllabically called an "associative array",
           which is a pretty good reason for simply calling it a "hash" instead.

       hash table
           A data structure used internally by Perl for implementing associative arrays (hashes) efficiently.  See
           also "bucket".

       header file
           A file containing certain required definitions that you must include "ahead" of the rest of your program to
           do certain obscure operations.  A C header file has a .h extension.  Perl doesn't really have header files,
           though historically Perl has sometimes used translated .h files with a .ph extension.  See "require" in
           perlfunc.  (Header files have been superseded by the "module" mechanism.)

       here document
           So called because of a similar construct in shells that pretends that the lines following the "command" are
           a separate "file" to be fed to the command, up to some terminating string.  In Perl, however, it's just a
           fancy form of quoting.

           A number in base 16, "hex" for short.  The digits for 10 through 16 are customarily represented by the let-
           ters "a" through "f".  Hexadecimal constants in Perl start with "0x".  See also "hex" in perlfunc.

       home directory
           The directory you are put into when you log in.  On a Unix system, the name is often placed into $ENV{HOME}
           or $ENV{LOGDIR} by login, but you can also find it with "(getpwuid($<))[7]".  (Some platforms do not have a
           concept of a home directory.)

           The computer on which a program or other data resides.

           Excessive pride, the sort of thing Zeus zaps you for.  Also the quality that makes you write (and maintain)
           programs that other people won't want to say bad things about.  Hence, the third great virtue of a program-
           mer.  See also "laziness" and "impatience".

       HV  Short for a "hash value" typedef, which holds Perl's internal representation of a hash.  The "HV" type is a
           subclass of "SV".


           A legally formed name for most anything in which a computer program might be interested.  Many languages
           (including Perl) allow identifiers that start with a letter and contain letters and digits.  Perl also
           counts the underscore character as a valid letter.  (Perl also has more complicated names, such as "quali-
           fied" names.)

           The anger you feel when the computer is being lazy.  This makes you write programs that don't just react to
           your needs, but actually anticipate them.  Or at least that pretend to.  Hence, the second great virtue of
           a programmer.  See also "laziness" and "hubris".

           How a piece of code actually goes about doing its job.  Users of the code should not count on implementa-
           tion details staying the same unless they are part of the published "interface".

           To gain access to symbols that are exported from another module.  See "use" in perlfunc.

           To increase the value of something by 1 (or by some other number, if so specified).

           In olden days, the act of looking up a "key" in an actual index (such as a phone book), but now merely the
           act of using any kind of key or position to find the corresponding "value", even if no index is involved.
           Things have degenerated to the point that Perl's index function merely locates the position (index) of one
           string in another.

       indirect filehandle
           An "expression" that evaluates to something that can be used as a "filehandle": a "string" (filehandle
           name), a "typeglob", a typeglob "reference", or a low-level "IO" object.

       indirect object
           In English grammar, a short noun phrase between a verb and its direct object indicating the beneficiary or
           recipient of the action.  In Perl, "print STDOUT "$foo\n";" can be understood as "verb indirect-object
           object" where "STDOUT" is the recipient of the print action, and "$foo" is the object being printed.  Simi-
           larly, when invoking a "method", you might place the invocant between the method and its arguments:

             $gollum = new Pathetic::Creature "Smeagol";
             give $gollum "Fisssssh!";
             give $gollum "Precious!";

       indirect object slot
           The syntactic position falling between a method call and its arguments when using the indirect object invo-
           cation syntax.  (The slot is distinguished by the absence of a comma between it and the next argument.)
           "STDERR" is in the indirect object slot here:

             print STDERR "Awake!  Awake!  Fear, Fire,
                 Foes!  Awake!\n";

           If something in a program isn't the value you're looking for but indicates where the value is, that's indi-
           rection.  This can be done with either symbolic references or hard references.

           An "operator" that comes in between its operands, such as multiplication in "24 * 7".

           What you get from your ancestors, genetically or otherwise.  If you happen to be a "class", your ancestors
           are called base classes and your descendants are called derived classes.  See "single inheritance" and
           "multiple inheritance".

           Short for "an instance of a class", meaning an "object" of that "class".

       instance variable
           An "attribute" of an "object"; data stored with the particular object rather than with the class as a

           A number with no fractional (decimal) part.  A counting number, like 1, 2, 3, and so on, but including 0
           and the negatives.

           The services a piece of code promises to provide forever, in contrast to its "implementation", which it
           should feel free to change whenever it likes.

           The insertion of a scalar or list value somewhere in the middle of another value, such that it appears to
           have been there all along.  In Perl, variable interpolation happens in double-quoted strings and patterns,
           and list interpolation occurs when constructing the list of values to pass to a list operator or other such
           construct that takes a "LIST".

           Strictly speaking, a program that reads a second program and does what the second program says directly
           without turning the program into a different form first, which is what compilers do.  Perl is not an inter-
           preter by this definition, because it contains a kind of compiler that takes a program and turns it into a
           more executable form (syntax trees) within the perl process itself, which the Perl "run time" system then

           The agent on whose behalf a "method" is invoked.  In a "class" method, the invocant is a package name.  In
           an "instance" method, the invocant is an object reference.

           The act of calling up a deity, daemon, program, method, subroutine, or function to get it do what you think
           it's supposed to do.  We usually "call" subroutines but "invoke" methods, since it sounds cooler.

       I/O Input from, or output to, a "file" or "device".

       IO  An internal I/O object.  Can also mean "indirect object".

       IP  Internet Protocol, or Intellectual Property.

       IPC Interprocess Communication.

           A relationship between two objects in which one object is considered to be a more specific version of the
           other, generic object: "A camel is a mammal."  Since the generic object really only exists in a Platonic
           sense, we usually add a little abstraction to the notion of objects and think of the relationship as being
           between a generic "base class" and a specific "derived class".  Oddly enough, Platonic classes don't always
           have Platonic relationships--see "inheritance".

           Doing something repeatedly.

           A special programming gizmo that keeps track of where you are in something that you're trying to iterate
           over.  The "foreach" loop in Perl contains an iterator; so does a hash, allowing you to each through it.

       IV  The integer four, not to be confused with six, Tom's favorite editor.  IV also means an internal Integer
           Value of the type a "scalar" can hold, not to be confused with an "NV".


           "Just Another Perl Hacker," a clever but cryptic bit of Perl code that when executed, evaluates to that
           string.  Often used to illustrate a particular Perl feature, and something of an ungoing Obfuscated Perl
           Contest seen in Usenix signatures.


       key The string index to a "hash", used to look up the "value" associated with that key.

           See "reserved words".


           A name you give to a "statement" so that you can talk about that statement elsewhere in the program.

           The quality that makes you go to great effort to reduce overall energy expenditure.  It makes you write
           labor-saving programs that other people will find useful, and document what you wrote so you don't have to
           answer so many questions about it.  Hence, the first great virtue of a programmer.  Also hence, this book.
           See also "impatience" and "hubris".

       left shift
           A "bit shift" that multiplies the number by some power of 2.

       leftmost longest
           The preference of the "regular expression" engine to match the leftmost occurrence of a "pattern", then
           given a position at which a match will occur, the preference for the longest match (presuming the use of a
           "greedy" quantifier).  See perlre for much more on this subject.

           Fancy term for a "token".

           Fancy term for a "tokener".

       lexical analysis
           Fancy term for "tokenizing".

       lexical scoping
           Looking at your Oxford English Dictionary through a microscope.  (Also known as "static scoping", because
           dictionaries don't change very fast.)  Similarly, looking at variables stored in a private dictionary
           (namespace) for each scope, which are visible only from their point of declaration down to the end of the
           lexical scope in which they are declared.  --Syn. "static scoping".  --Ant. "dynamic scoping".

       lexical variable
           A "variable" subject to "lexical scoping", declared by my.  Often just called a "lexical".  (The our decla-
           ration declares a lexically scoped name for a global variable, which is not itself a lexical variable.)

           Generally, a collection of procedures.  In ancient days, referred to a collection of subroutines in a .pl
           file.  In modern times, refers more often to the entire collection of Perl modules on your system.

           Last In, First Out.  See also "FIFO".  A LIFO is usually called a "stack".

           In Unix, a sequence of zero or more non-newline characters terminated with a "newline" character.  On non-
           Unix machines, this is emulated by the C library even if the underlying "operating system" has different

       line buffering
           Used by a "standard I/O" output stream that flushes its "buffer" after every "newline".  Many standard I/O
           libraries automatically set up line buffering on output that is going to the terminal.

       line number
           The number of lines read previous to this one, plus 1.  Perl keeps a separate line number for each source
           or input file it opens.  The current source file's line number is represented by "__LINE__".  The current
           input line number (for the file that was most recently read via "<FH>") is represented by the $.
           ($INPUT_LINE_NUMBER) variable.  Many error messages report both values, if available.

           Used as a noun, a name in a "directory", representing a "file".  A given file can have multiple links to
           it.  It's like having the same phone number listed in the phone directory under different names.  As a
           verb, to resolve a partially compiled file's unresolved symbols into a (nearly) executable image.  Linking
           can generally be static or dynamic, which has nothing to do with static or dynamic scoping.

           A syntactic construct representing a comma-separated list of expressions, evaluated to produce a "list
           value".  Each "expression" in a "LIST" is evaluated in "list context" and interpolated into the list value.

           An ordered set of scalar values.

       list context
           The situation in which an "expression" is expected by its surroundings (the code calling it) to return a
           list of values rather than a single value.  Functions that want a "LIST" of arguments tell those arguments
           that they should produce a list value.  See also "context".

       list operator
           An "operator" that does something with a list of values, such as join or grep.  Usually used for named
           built-in operators (such as print, unlink, and system) that do not require parentheses around their "argu-
           ment" list.

       list value
           An unnamed list of temporary scalar values that may be passed around within a program from any list-gener-
           ating function to any function or construct that provides a "list context".

           A token in a programming language such as a number or "string" that gives you an actual "value" instead of
           merely representing possible values as a "variable" does.

           From Swift: someone who eats eggs little end first.  Also used of computers that store the least signifi-
           cant "byte" of a word at a lower byte address than the most significant byte.  Often considered superior to
           big-endian machines.  See also "big-endian".

           Not meaning the same thing everywhere.  A global variable in Perl can be localized inside a dynamic scope
           via the local operator.

       logical operator
           Symbols representing the concepts "and", "or", "xor", and "not".

           An "assertion" that peeks at the string to the right of the current match location.

           An "assertion" that peeks at the string to the left of the current match location.

           A construct that performs something repeatedly, like a roller coaster.

       loop control statement
           Any statement within the body of a loop that can make a loop prematurely stop looping or skip an "itera-
           tion".  Generally you shouldn't try this on roller coasters.

       loop label
           A kind of key or name attached to a loop (or roller coaster) so that loop control statements can talk about
           which loop they want to control.

           Able to serve as an "lvalue".

           Term used by language lawyers for a storage location you can assign a new "value" to, such as a "variable"
           or an element of an "array".  The "l" is short for "left", as in the left side of an assignment, a typical
           place for lvalues.  An "lvaluable" function or expression is one to which a value may be assigned, as in
           "pos($x) = 10".

       lvalue modifier
           An adjectival pseudofunction that warps the meaning of an "lvalue" in some declarative fashion.  Currently
           there are three lvalue modifiers: my, our, and local.


           Technically speaking, any extra semantics attached to a variable such as $!, $0, %ENV, or %SIG, or to any
           tied variable.  Magical things happen when you diddle those variables.

       magical increment
           An "increment" operator that knows how to bump up alphabetics as well as numbers.

       magical variables
           Special variables that have side effects when you access them or assign to them.  For example, in Perl,
           changing elements of the %ENV array also changes the corresponding environment variables that subprocesses
           will use.  Reading the $! variable gives you the current system error number or message.

           A file that controls the compilation of a program.  Perl programs don't usually need a "Makefile" because
           the Perl compiler has plenty of self-control.

       man The Unix program that displays online documentation (manual pages) for you.

           A "page" from the manuals, typically accessed via the man(1) command.  A manpage contains a SYNOPSIS, a
           DESCRIPTION, a list of BUGS, and so on, and is typically longer than a page.  There are manpages document-
           ing commands, syscalls, "library" functions, devices, protocols, files, and such.  In this book, we call
           any piece of standard Perl documentation (like perlop or perldelta) a manpage, no matter what format it's
           installed in on your system.

           See "pattern matching".

       member data
           See "instance variable".

           This always means your main memory, not your disk.  Clouding the issue is the fact that your machine may
           implement "virtual" memory; that is, it will pretend that it has more memory than it really does, and it'll
           use disk space to hold inactive bits.  This can make it seem like you have a little more memory than you
           really do, but it's not a substitute for real memory.  The best thing that can be said about virtual memory
           is that it lets your performance degrade gradually rather than suddenly when you run out of real memory.
           But your program can die when you run out of virtual memory too, if you haven't thrashed your disk to death

           A "character" that is not supposed to be treated normally.  Which characters are to be treated specially as
           metacharacters varies greatly from context to context.  Your "shell" will have certain metacharacters, dou-
           ble-quoted Perl strings have other metacharacters, and "regular expression" patterns have all the double-
           quote metacharacters plus some extra ones of their own.

           Something we'd call a "metacharacter" except that it's a sequence of more than one character.  Generally,
           the first character in the sequence must be a true metacharacter to get the other characters in the meta-
           symbol to misbehave along with it.

           A kind of action that an "object" can take if you tell it to.  See perlobj.

           The belief that "small is beautiful."  Paradoxically, if you say something in a small language, it turns
           out big, and if you say it in a big language, it turns out small.  Go figure.

           In the context of the stat syscall, refers to the field holding the "permission bits" and the type of the

           See "statement modifier", "regular expression modifier", and "lvalue modifier", not necessarily in that

           A "file" that defines a "package" of (almost) the same name, which can either "export" symbols or function
           as an "object" class.  (A module's main .pm file may also load in other files in support of the module.)
           See the use built-in.

           An integer divisor when you're interested in the remainder instead of the quotient.

           Short for Perl Monger, a purveyor of Perl.

           A temporary value scheduled to die when the current statement finishes.

       multidimensional array
           An array with multiple subscripts for finding a single element.  Perl implements these using refer-
           ences--see perllol and perldsc.

       multiple inheritance
           The features you got from your mother and father, mixed together unpredictably.  (See also "inheritance",
           and "single inheritance".)  In computer languages (including Perl), the notion that a given class may have
           multiple direct ancestors or base classes.


       named pipe
           A "pipe" with a name embedded in the "filesystem" so that it can be accessed by two unrelated processes.

           A domain of names.  You needn't worry about whether the names in one such domain have been used in another.
           See "package".

       network address
           The most important attribute of a socket, like your telephone's telephone number.  Typically an IP address.
           See also "port".

           A single character that represents the end of a line, with the ASCII value of 012 octal under Unix (but 015
           on a Mac), and represented by "\n" in Perl strings.  For Windows machines writing text files, and for cer-
           tain physical devices like terminals, the single newline gets automatically translated by your C library
           into a line feed and a carriage return, but normally, no translation is done.

       NFS Network File System, which allows you to mount a remote filesystem as if it were local.

       null character
           A character with the ASCII value of zero.  It's used by C to terminate strings, but Perl allows strings to
           contain a null.

       null list
           A "list value" with zero elements, represented in Perl by "()".

       null string
           A "string" containing no characters, not to be confused with a string containing a "null character", which
           has a positive length and is "true".

       numeric context
           The situation in which an expression is expected by its surroundings (the code calling it) to return a num-
           ber.  See also "context" and "string context".

       NV  Short for Nevada, no part of which will ever be confused with civilization.  NV also means an internal
           floating-point Numeric Value of the type a "scalar" can hold, not to be confused with an "IV".

           Half a "byte", equivalent to one "hexadecimal" digit, and worth four bits.


           An "instance" of a "class".  Something that "knows" what user-defined type (class) it is, and what it can
           do because of what class it is.  Your program can request an object to do things, but the object gets to
           decide whether it wants to do them or not.  Some objects are more accommodating than others.

           A number in base 8.  Only the digits 0 through 7 are allowed.  Octal constants in Perl start with 0, as in
           013.  See also the oct function.

           How many things you have to skip over when moving from the beginning of a string or array to a specific
           position within it.  Thus, the minimum offset is zero, not one, because you don't skip anything to get to
           the first item.

           An entire computer program crammed into one line of text.

       open source software
           Programs for which the source code is freely available and freely redistributable, with no commercial
           strings attached.  For a more detailed definition, see <>;.

           An "expression" that yields a "value" that an "operator" operates on.  See also "precedence".

       operating system
           A special program that runs on the bare machine and hides the gory details of managing processes and
           devices.  Usually used in a looser sense to indicate a particular culture of programming.  The loose sense
           can be used at varying levels of specificity.  At one extreme, you might say that all versions of Unix and
           Unix-lookalikes are the same operating system (upsetting many people, especially lawyers and other advo-
           cates).  At the other extreme, you could say this particular version of this particular vendor's operating
           system is different from any other version of this or any other vendor's operating system.  Perl is much
           more portable across operating systems than many other languages.  See also "architecture" and "platform".

           A gizmo that transforms some number of input values to some number of output values, often built into a
           language with a special syntax or symbol.  A given operator may have specific expectations about what types
           of data you give as its arguments (operands) and what type of data you want back from it.

       operator overloading
           A kind of "overloading" that you can do on built-in operators to make them work on objects as if the
           objects were ordinary scalar values, but with the actual semantics supplied by the object class.  This is
           set up with the overload "pragma".

           See either switches or "regular expression modifier".

           Giving additional meanings to a symbol or construct.  Actually, all languages do overloading to one extent
           or another, since people are good at figuring out things from "context".

           Hiding or invalidating some other definition of the same name.  (Not to be confused with "overloading",
           which adds definitions that must be disambiguated some other way.) To confuse the issue further, we use the
           word with two overloaded definitions: to describe how you can define your own "subroutine" to hide a built-
           in "function" of the same name (see "Overriding Built-in Functions" in perlsub) and to describe how you can
           define a replacement "method" in a "derived class" to hide a "base class"'s method of the same name (see

           The one user (apart from the superuser) who has absolute control over a "file".  A file may also have a
           "group" of users who may exercise joint ownership if the real owner permits it.  See "permission bits".


           A "namespace" for global variables, subroutines, and the like, such that they can be kept separate from
           like-named symbols in other namespaces.  In a sense, only the package is global, since the symbols in the
           package's symbol table are only accessible from code compiled outside the package by naming the package.
           But in another sense, all package symbols are also globals--they're just well-organized globals.

       pad Short for "scratchpad".

           See "argument".

       parent class
           See "base class".

       parse tree
           See "syntax tree".

           The subtle but sometimes brutal art of attempting to turn your possibly malformed program into a valid
           "syntax tree".

           To fix by applying one, as it were.  In the realm of hackerdom, a listing of the differences between two
           versions of a program as might be applied by the patch(1) program when you want to fix a bug or upgrade
           your old version.

           The list of directories the system searches to find a program you want to "execute".  The list is stored as
           one of your environment variables, accessible in Perl as $ENV{PATH}.

           A fully qualified filename such as /usr/bin/perl.  Sometimes confused with "PATH".

           A template used in "pattern matching".

       pattern matching
           Taking a pattern, usually a "regular expression", and trying the pattern various ways on a string to see
           whether there's any way to make it fit.  Often used to pick interesting tidbits out of a file.

       permission bits
           Bits that the "owner" of a file sets or unsets to allow or disallow access to other people.  These flag
           bits are part of the "mode" word returned by the stat built-in when you ask about a file.  On Unix systems,
           you can check the ls(1) manpage for more information.

           What you get when you do "Perl++" twice.  Doing it only once will curl your hair.  You have to increment it
           eight times to shampoo your hair.  Lather, rinse, iterate.

           A direct "connection" that carries the output of one "process" to the input of another without an interme-
           diate temporary file.  Once the pipe is set up, the two processes in question can read and write as if they
           were talking to a normal file, with some caveats.

           A series of processes all in a row, linked by pipes, where each passes its output stream to the next.

           The entire hardware and software context in which a program runs.  A
            program written in a platform-dependent language might break if you change any of: machine, operating sys-
           tem, libraries, compiler, or system configuration.  The perl interpreter has to be compiled differently for
           each platform because it is implemented in C, but programs written in the Perl language are largely plat-

       pod The markup used to embed documentation into your Perl code.  See perlpod.

           A "variable" in a language like C that contains the exact memory location of some other item.  Perl handles
           pointers internally so you don't have to worry about them.  Instead, you just use symbolic pointers in the
           form of keys and "variable" names, or hard references, which aren't pointers (but act like pointers and do
           in fact contain pointers).

           The notion that you can tell an "object" to do something generic, and the object will interpret the command
           in different ways depending on its type.  [<Gk many shapes]

           The part of the address of a TCP or UDP socket that directs packets to the correct process after finding
           the right machine, something like the phone extension you give when you reach the company operator.  Also,
           the result of converting code to run on a different platform than originally intended, or the verb denoting
           this conversion.

           Once upon a time, C code compilable under both BSD and SysV.  In general, code that can be easily converted
           to run on another "platform", where "easily" can be defined however you like, and usually is.  Anything may
           be considered portable if you try hard enough.  See mobile home or London Bridge.

           Someone who "carries" software from one "platform" to another.  Porting programs written in platform-depen-
           dent languages such as C can be difficult work, but porting programs like Perl is very much worth the

           The Portable Operating System Interface specification.

           An "operator" that follows its "operand", as in "$x++".

       pp  An internal shorthand for a "push-pop" code, that is, C code implementing Perl's stack machine.

           A standard module whose practical hints and suggestions are received (and possibly ignored) at compile
           time.  Pragmas are named in all lowercase.

           The rules of conduct that, in the absence of other guidance, determine what should happen first.  For exam-
           ple, in the absence of parentheses, you always do multiplication before addition.

           An "operator" that precedes its "operand", as in "++$x".

           What some helper "process" did to transform the incoming data into a form more suitable for the current
           process.  Often done with an incoming "pipe".  See also "C preprocessor".

           A "subroutine".

           An instance of a running program.  Under multitasking systems like Unix, two or more separate processes
           could be running the same program independently at the same time--in fact, the fork function is designed to
           bring about this happy state of affairs.  Under other operating systems, processes are sometimes called
           "threads", "tasks", or "jobs", often with slight nuances in meaning.

       program generator
           A system that algorithmically writes code for you in a high-level language.  See also "code generator".

       progressive matching
           Pattern matching that picks up where it left off before.

           See either "instance variable" or "character property".

           In networking, an agreed-upon way of sending messages back and forth so that neither correspondent will get
           too confused.

           An optional part of a "subroutine" declaration telling the Perl compiler how many and what flavor of argu-
           ments may be passed as "actual arguments", so that you can write subroutine calls that parse much like
           built-in functions.  (Or don't parse, as the case may be.)

           A construct that sometimes looks like a function but really isn't.  Usually reserved for "lvalue" modifiers
           like my, for "context" modifiers like scalar, and for the pick-your-own-quotes constructs, "q//", "qq//",
           "qx//", "qw//", "qr//", "m//", "s///", "y///", and "tr///".

           A reference to an array whose initial element happens to hold a reference to a hash.  You can treat a pseu-
           dohash reference as either an array reference or a hash reference.

           An "operator" that looks something like a "literal", such as the output-grabbing operator, "'""command""'".

       public domain
           Something not owned by anybody.  Perl is copyrighted and is thus not in the public domain--it's just
           "freely available" and "freely redistributable".

           A notional "baton" handed around the Perl community indicating who is the lead integrator in some arena of

           A "pumpkin" holder, the person in charge of pumping the pump, or at least priming it.  Must be willing to
           play the part of the Great Pumpkin now and then.

       PV  A "pointer value", which is Perl Internals Talk for a "char*".


           Possessing a complete name.  The symbol $Ent::moot is qualified; $moot is unqualified.  A fully qualified
           filename is specified from the top-level directory.

           A component of a "regular expression" specifying how many times the foregoing "atom" may occur.


           With respect to files, one that has the proper permission bit set to let you access the file.  With respect
           to computer programs, one that's written well enough that someone has a chance of figuring out what it's
           trying to do.

           The last rites performed by a parent "process" on behalf of a deceased child process so that it doesn't
           remain a "zombie".  See the wait and waitpid function calls.

           A set of related data values in a "file" or "stream", often associated with a unique "key" field.  In Unix,
           often commensurate with a "line", or a blank-line-terminated set of lines (a "paragraph").  Each line of
           the /etc/passwd file is a record, keyed on login name, containing information about that user.

           The art of defining something (at least partly) in terms of itself, which is a naughty no-no in dictionar-
           ies but often works out okay in computer programs if you're careful not to recurse forever, which is like
           an infinite loop with more spectacular failure modes.

           Where you look to find a pointer to information somewhere else.  (See "indirection".)  References come in
           two flavors, symbolic references and hard references.

           Whatever a reference refers to, which may or may not have a name.  Common types of referents include
           scalars, arrays, hashes, and subroutines.

           See "regular expression".

       regular expression
           A single entity with various interpretations, like an elephant.  To a computer scientist, it's a grammar
           for a little language in which some strings are legal and others aren't.  To normal people, it's a pattern
           you can use to find what you're looking for when it varies from case to case.  Perl's regular expressions
           are far from regular in the theoretical sense, but in regular use they work quite well.  Here's a regular
           expression: "/Oh s.*t./".  This will match strings like ""Oh say can you see by the dawn's early light""
           and ""Oh sit!"".  See perlre.

       regular expression modifier
           An option on a pattern or substitution, such as "/i" to render the pattern case insensitive.  See also

       regular file
           A "file" that's not a "directory", a "device", a named "pipe" or "socket", or a "symbolic link".  Perl uses
           the "-f" file test operator to identify regular files.  Sometimes called a "plain" file.

       relational operator
           An "operator" that says whether a particular ordering relationship is "true" about a pair of operands.
           Perl has both numeric and string relational operators.  See "collating sequence".

       reserved words
           A word with a specific, built-in meaning to a "compiler", such as "if" or delete.  In many languages (not
           Perl), it's illegal to use reserved words to name anything else.  (Which is why they're reserved, after
           all.)  In Perl, you just can't use them to name labels or filehandles.  Also called "keywords".

       return value
           The "value" produced by a "subroutine" or "expression" when evaluated.  In Perl, a return value may be
           either a "list" or a "scalar".

       RFC Request For Comment, which despite the timid connotations is the name of a series of important standards

       right shift
           A "bit shift" that divides a number by some power of 2.

           The superuser (UID == 0).  Also, the top-level directory of the filesystem.

           What you are told when someone thinks you should Read The Fine Manual.

       run phase
           Any time after Perl starts running your main program.  See also "compile phase".  Run phase is mostly spent
           in "run time" but may also be spent in "compile time" when require, do "FILE", or eval "STRING" operators
           are executed or when a substitution uses the "/ee" modifier.

       run time
           The time when Perl is actually doing what your code says to do, as opposed to the earlier period of time
           when it was trying to figure out whether what you said made any sense whatsoever, which is "compile time".

       run-time pattern
           A pattern that contains one or more variables to be interpolated before parsing the pattern as a "regular
           expression", and that therefore cannot be analyzed at compile time, but must be re-analyzed each time the
           pattern match operator is evaluated.  Run-time patterns are useful but expensive.

       RV  A recreational vehicle, not to be confused with vehicular recreation.  RV also means an internal Reference
           Value of the type a "scalar" can hold.  See also "IV" and "NV" if you're not confused yet.

           A "value" that you might find on the right side of an "assignment".  See also "lvalue".


           A simple, singular value; a number, "string", or "reference".

       scalar context
           The situation in which an "expression" is expected by its surroundings (the code calling it) to return a
           single "value" rather than a "list" of values.  See also "context" and "list context".  A scalar context
           sometimes imposes additional constraints on the return value--see "string context" and "numeric context".
           Sometimes we talk about a "Boolean context" inside conditionals, but this imposes no additional con-
           straints, since any scalar value, whether numeric or "string", is already true or false.

       scalar literal
           A number or quoted "string"--an actual "value" in the text of your program, as opposed to a "variable".

       scalar value
           A value that happens to be a "scalar" as opposed to a "list".

       scalar variable
           A "variable" prefixed with "$" that holds a single value.

           How far away you can see a variable from, looking through one.  Perl has two visibility mechanisms: it does
           "dynamic scoping" of local variables, meaning that the rest of the "block", and any subroutines that are
           called by the rest of the block, can see the variables that are local to the block.  Perl does "lexical
           scoping" of my variables, meaning that the rest of the block can see the variable, but other subroutines
           called by the block cannot see the variable.

           The area in which a particular invocation of a particular file or subroutine keeps some of its temporary
           values, including any lexically scoped variables.

           A text "file" that is a program intended to be executed directly rather than compiled to another form of
           file before execution.  Also, in the context of "Unicode", a writing system for a particular language or
           group of languages, such as Greek, Bengali, or Klingon.

       script kiddie
           A "cracker" who is not a "hacker", but knows just enough to run canned scripts.  A cargo-cult programmer.

       sed A venerable Stream EDitor from which Perl derives some of its ideas.

           A fancy kind of interlock that prevents multiple threads or processes from using up the same resources

           A "character" or "string" that keeps two surrounding strings from being confused with each other.  The
           split function works on separators.  Not to be confused with delimiters or terminators.  The "or" in the
           previous sentence separated the two alternatives.

           Putting a fancy "data structure" into linear order so that it can be stored as a "string" in a disk file or
           database or sent through a "pipe".  Also called marshalling.

           In networking, a "process" that either advertises a "service" or just hangs around at a known location and
           waits for clients who need service to get in touch with it.

           Something you do for someone else to make them happy, like giving them the time of day (or of their life).
           On some machines, well-known services are listed by the getservent function.

           Same as "setuid", only having to do with giving away "group" privileges.

           Said of a program that runs with the privileges of its "owner" rather than (as is usually the case) the
           privileges of whoever is running it.  Also describes the bit in the mode word ("permission bits") that con-
           trols the feature.  This bit must be explicitly set by the owner to enable this feature, and the program
           must be carefully written not to give away more privileges than it ought to.

       shared memory
           A piece of "memory" accessible by two different processes who otherwise would not see each other's memory.

           Irish for the whole McGillicuddy.  In Perl culture, a portmanteau of "sharp" and "bang", meaning the "#!"
           sequence that tells the system where to find the interpreter.

           A "command"-line "interpreter".  The program that interactively gives you a prompt, accepts one or more
           lines of input, and executes the programs you mentioned, feeding each of them their proper arguments and
           input data.  Shells can also execute scripts containing such commands.  Under Unix, typical shells include
           the Bourne shell (/bin/sh), the C shell (/bin/csh), and the Korn shell (/bin/ksh).  Perl is not strictly a
           shell because it's not interactive (although Perl programs can be interactive).

       side effects
           Something extra that happens when you evaluate an "expression".  Nowadays it can refer to almost anything.
           For example, evaluating a simple assignment statement typically has the "side effect" of assigning a value
           to a variable.  (And you thought assigning the value was your primary intent in the first place!)  Like-
           wise, assigning a value to the special variable $| ($AUTOFLUSH) has the side effect of forcing a flush
           after every write or print on the currently selected filehandle.

           A bolt out of the blue; that is, an event triggered by the "operating system", probably when you're least
           expecting it.

       signal handler
           A "subroutine" that, instead of being content to be called in the normal fashion, sits around waiting for a
           bolt out of the blue before it will deign to "execute".  Under Perl, bolts out of the blue are called sig-
           nals, and you send them with the kill built-in.  See "%SIG" in perlvar and "Signals" in perlipc.

       single inheritance
           The features you got from your mother, if she told you that you don't have a father.  (See also "inheri-
           tance" and "multiple inheritance".)  In computer languages, the notion that classes reproduce asexually so
           that a given class can only have one direct ancestor or "base class".  Perl supplies no such restriction,
           though you may certainly program Perl that way if you like.

           A selection of any number of elements from a "list", "array", or "hash".

           To read an entire "file" into a "string" in one operation.

           An endpoint for network communication among multiple processes that works much like a telephone or a post
           office box.  The most important thing about a socket is its "network address" (like a phone number).  Dif-
           ferent kinds of sockets have different kinds of addresses--some look like filenames, and some don't.

       soft reference
           See "symbolic reference".

       source filter
           A special kind of "module" that does "preprocessing" on your script just before it gets to the "tokener".

           A device you can put things on the top of, and later take them back off in the opposite order in which you
           put them on.  See "LIFO".

           Included in the official Perl distribution, as in a standard module, a standard tool, or a standard Perl

       standard error
           The default output "stream" for nasty remarks that don't belong in "standard output".  Represented within a
           Perl program by the "filehandle" "STDERR".  You can use this stream explicitly, but the die and warn built-
           ins write to your standard error stream automatically.

       standard I/O
           A standard C library for doing buffered input and output to the "operating system".  (The "standard" of
           standard I/O is only marginally related to the "standard" of standard input and output.)  In general, Perl
           relies on whatever implementation of standard I/O a given operating system supplies, so the buffering char-
           acteristics of a Perl program on one machine may not exactly match those on another machine.  Normally this
           only influences efficiency, not semantics.  If your standard I/O package is doing block buffering and you
           want it to "flush" the buffer more often, just set the $| variable to a true value.

       standard input
           The default input "stream" for your program, which if possible shouldn't care where its data is coming
           from.  Represented within a Perl program by the "filehandle" "STDIN".

       standard output
           The default output "stream" for your program, which if possible shouldn't care where its data is going.
           Represented within a Perl program by the "filehandle" "STDOUT".

       stat structure
           A special internal spot in which Perl keeps the information about the last "file" on which you requested

           A "command" to the computer about what to do next, like a step in a recipe: "Add marmalade to batter and
           mix until mixed."  A statement is distinguished from a "declaration", which doesn't tell the computer to do
           anything, but just to learn something.

       statement modifier
           A "conditional" or "loop" that you put after the "statement" instead of before, if you know what we mean.

           Varying slowly compared to something else.  (Unfortunately, everything is relatively stable compared to
           something else, except for certain elementary particles, and we're not so sure about them.)  In computers,
           where things are supposed to vary rapidly, "static" has a derogatory connotation, indicating a slightly
           dysfunctional "variable", "subroutine", or "method".  In Perl culture, the word is politely avoided.

       static method
           No such thing.  See "class method".

       static scoping
           No such thing.  See "lexical scoping".

       static variable
           No such thing.  Just use a "lexical variable" in a scope larger than your "subroutine".

           The "value" returned to the parent "process" when one of its child processes dies.  This value is placed in
           the special variable $?.  Its upper eight bits are the exit status of the defunct process, and its lower
           eight bits identify the signal (if any) that the process died from.  On Unix systems, this status value is
           the same as the status word returned by wait(2).  See "system" in perlfunc.

           See "standard error".

           See "standard input".

           See "standard I/O".

           See "standard output".

           A flow of data into or out of a process as a steady sequence of bytes or characters, without the appearance
           of being broken up into packets.  This is a kind of "interface"--the underlying "implementation" may well
           break your data up into separate packets for delivery, but this is hidden from you.

           A sequence of characters such as "He said !@#*&%@#*?!".  A string does not have to be entirely printable.

       string context
           The situation in which an expression is expected by its surroundings (the code calling it) to return a
           "string".  See also "context" and "numeric context".

           The process of producing a "string" representation of an abstract object.

           C keyword introducing a structure definition or name.

           See "data structure".

           See "derived class".

           A component of a "regular expression" pattern.

           A named or otherwise accessible piece of program that can be invoked from elsewhere in the program in order
           to accomplish some sub-goal of the program.  A subroutine is often parameterized to accomplish different
           but related things depending on its input arguments.  If the subroutine returns a meaningful "value", it is
           also called a "function".

           A "value" that indicates the position of a particular "array" "element" in an array.

           Changing parts of a string via the "s///" operator.  (We avoid use of this term to mean "variable interpo-

           A portion of a "string", starting at a certain "character" position ("offset") and proceeding for a certain
           number of characters.

           See "base class".

           The person whom the "operating system" will let do almost anything.  Typically your system administrator or
           someone pretending to be your system administrator.  On Unix systems, the "root" user.  On Windows systems,
           usually the Administrator user.

       SV  Short for "scalar value".  But within the Perl interpreter every "referent" is treated as a member of a
           class derived from SV, in an object-oriented sort of way.  Every "value" inside Perl is passed around as a
           C language "SV*" pointer.  The SV "struct" knows its own "referent type", and the code is smart enough (we
           hope) not to try to call a "hash" function on a "subroutine".

           An option you give on a command line to influence the way your program works, usually introduced with a
           minus sign.  The word is also used as a nickname for a "switch statement".

       switch cluster
           The combination of multiple command-line switches (e.g., -a -b -c) into one switch (e.g., -abc).  Any
           switch with an additional "argument" must be the last switch in a cluster.

       switch statement
           A program technique that lets you evaluate an "expression" and then, based on the value of the expression,
           do a multiway branch to the appropriate piece of code for that value.  Also called a "case structure",
           named after the similar Pascal construct.  Most switch statements in Perl are spelled "for".  See "Basic
           BLOCKs and Switch Statements" in perlsyn.

           Generally, any "token" or "metasymbol".  Often used more specifically to mean the sort of name you might
           find in a "symbol table".

       symbol table
           Where a "compiler" remembers symbols.  A program like Perl must somehow remember all the names of all the
           variables, filehandles, and subroutines you've used.  It does this by placing the names in a symbol table,
           which is implemented in Perl using a "hash table".  There is a separate symbol table for each "package" to
           give each package its own "namespace".

       symbolic debugger
           A program that lets you step through the execution of your program, stopping or printing things out here
           and there to see whether anything has gone wrong, and if so, what.  The "symbolic" part just means that you
           can talk to the debugger using the same symbols with which your program is written.

       symbolic link
           An alternate filename that points to the real "filename", which in turn points to the real "file".  When-
           ever the "operating system" is trying to parse a "pathname" containing a symbolic link, it merely substi-
           tutes the new name and continues parsing.

       symbolic reference
           A variable whose value is the name of another variable or subroutine.  By dereferencing the first variable,
           you can get at the second one.  Symbolic references are illegal under use strict 'refs'.

           Programming in which the orderly sequence of events can be determined; that is, when things happen one
           after the other, not at the same time.

       syntactic sugar
           An alternative way of writing something more easily; a shortcut.

           From Greek, "with-arrangement".  How things (particularly symbols) are put together with each other.

       syntax tree
           An internal representation of your program wherein lower-level constructs dangle off the higher-level con-
           structs enclosing them.

           A "function" call directly to the "operating system".  Many of the important subroutines and functions you
           use aren't direct system calls, but are built up in one or more layers above the system call level.  In
           general, Perl programmers don't need to worry about the distinction.  However, if you do happen to know
           which Perl functions are really syscalls, you can predict which of these will set the $!  ($ERRNO) variable
           on failure.  Unfortunately, beginning programmers often confusingly employ the term "system call" to mean
           what happens when you call the Perl system function, which actually involves many syscalls.  To avoid any
           confusion, we nearly always use say "syscall" for something you could call indirectly via Perl's syscall
           function, and never for something you would call with Perl's system function.


           Said of data derived from the grubby hands of a user and thus unsafe for a secure program to rely on.  Perl
           does taint checks if you run a "setuid" (or "setgid") program, or if you use the -T switch.

       TCP Short for Transmission Control Protocol.  A protocol wrapped around the Internet Protocol to make an unre-
           liable packet transmission mechanism appear to the application program to be a reliable "stream" of bytes.

           Short for a "terminal", that is, a leaf node of a "syntax tree".  A thing that functions grammatically as
           an "operand" for the operators in an expression.

           A "character" or "string" that marks the end of another string.  The $/ variable contains the string that
           terminates a readline operation, which chomp deletes from the end.  Not to be confused with delimiters or
           separators.  The period at the end of this sentence is a terminator.

           An "operator" taking three operands.  Sometimes pronounced "trinary".

           A "string" or "file" containing primarily printable characters.

           Like a forked process, but without "fork"'s inherent memory protection.  A thread is lighter weight than a
           full process, in that a process could have multiple threads running around in it, all fighting over the
           same process's memory space unless steps are taken to protect threads from each other.  See threads.

       tie The bond between a magical variable and its implementation class.  See "tie" in perlfunc and perltie.

           There's More Than One Way To Do It, the Perl Motto.  The notion that there can be more than one valid path
           to solving a programming problem in context.  (This doesn't mean that more ways are always better or that
           all possible paths are equally desirable--just that there need not be One True Way.)  Pronounced TimToady.

           A morpheme in a programming language, the smallest unit of text with semantic significance.

           A module that breaks a program text into a sequence of tokens for later analysis by a parser.

           Splitting up a program text into tokens.  Also known as "lexing", in which case you get "lexemes" instead
           of tokens.

       toolbox approach
           The notion that, with a complete set of simple tools that work well together, you can build almost anything
           you want.  Which is fine if you're assembling a tricycle, but if you're building a defranishizing comboflux
           regurgalator, you really want your own machine shop in which to build special tools.  Perl is sort of a
           machine shop.

           To turn one string representation into another by mapping each character of the source string to its corre-
           sponding character in the result string.  See "tr/SEARCHLIST/REPLACEMENTLIST/cds" in perlop.

           An event that causes a "handler" to be run.

           Not a stellar system with three stars, but an "operator" taking three operands.  Sometimes pronounced

           A venerable typesetting language from which Perl derives the name of its $% variable and which is secretly
           used in the production of Camel books.

           Any scalar value that doesn't evaluate to 0 or "".

           Emptying a file of existing contents, either automatically when opening a file for writing or explicitly
           via the truncate function.

           See "data type" and "class".

       type casting
           Converting data from one type to another.  C permits this.  Perl does not need it.  Nor want it.

       typed lexical
           A "lexical variable" that is declared with a "class" type: "my Pony $bill".

           A type definition in the C language.

           Use of a single identifier, prefixed with "*".  For example, *name stands for any or all of $name, @name,
           %name, &name, or just "name".  How you use it determines whether it is interpreted as all or only one of
           them.  See "Typeglobs and Filehandles" in perldata.

           A description of how C types may be transformed to and from Perl types within an "extension" module written
           in "XS".


       UDP User Datagram Protocol, the typical way to send datagrams over the Internet.

       UID A user ID.  Often used in the context of "file" or "process" ownership.

           A mask of those "permission bits" that should be forced off when creating files or directories, in order to
           establish a policy of whom you'll ordinarily deny access to.  See the umask function.

       unary operator
           An operator with only one "operand", like "!" or chdir.  Unary operators are usually prefix operators; that
           is, they precede their operand.  The "++" and "--" operators can be either prefix or postfix.  (Their posi-
           tion does change their meanings.)

           A character set comprising all the major character sets of the world, more or less.  See <http://www.uni-

           A very large and constantly evolving language with several alternative and largely incompatible syntaxes,
           in which anyone can define anything any way they choose, and usually do.  Speakers of this language think
           it's easy to learn because it's so easily twisted to one's own ends, but dialectical differences make
           tribal intercommunication nearly impossible, and travelers are often reduced to a pidgin-like subset of the
           language.  To be universally understood, a Unix shell programmer must spend years of study in the art.
           Many have abandoned this discipline and now communicate via an Esperanto-like language called Perl.

           In ancient times, Unix was also used to refer to some code that a couple of people at Bell Labs wrote to
           make use of a PDP-7 computer that wasn't doing much of anything else at the time.


           An actual piece of data, in contrast to all the variables, references, keys, indexes, operators, and what-
           not that you need to access the value.

           A named storage location that can hold any of various kinds of "value", as your program sees fit.

       variable interpolation
           The "interpolation" of a scalar or array variable into a string.

           Said of a "function" that happily receives an indeterminate number of "actual arguments".

           Mathematical jargon for a list of scalar values.

           Providing the appearance of something without the reality, as in: virtual memory is not real memory.  (See
           also "memory".)  The opposite of "virtual" is "transparent", which means providing the reality of something
           without the appearance, as in: Perl handles the variable-length UTF-8 character encoding transparently.

       void context
           A form of "scalar context" in which an "expression" is not expected to return any "value" at all and is
           evaluated for its "side effects" alone.

           A "version" or "vector" "string" specified with a "v" followed by a series of decimal integers in dot nota-
           tion, for instance, "v1.20.300.4000".  Each number turns into a "character" with the specified ordinal
           value.  (The "v" is optional when there are at least three integers.)


           A message printed to the "STDERR" stream to the effect that something might be wrong but isn't worth blow-
           ing up over.  See "warn" in perlfunc and the warnings pragma.

       watch expression
           An expression which, when its value changes, causes a breakpoint in the Perl debugger.

           A "character" that moves your cursor but doesn't otherwise put anything on your screen.  Typically refers
           to any of: space, tab, line feed, carriage return, or form feed.

           In normal "computerese", the piece of data of the size most efficiently handled by your computer, typically
           32 bits or so, give or take a few powers of 2.  In Perl culture, it more often refers to an alphanumeric
           "identifier" (including underscores), or to a string of nonwhitespace characters bounded by whitespace or
           string boundaries.

       working directory
           Your current "directory", from which relative pathnames are interpreted by the "operating system".  The
           operating system knows your current directory because you told it with a chdir or because you started out
           in the place where your parent "process" was when you were born.

           A program or subroutine that runs some other program or subroutine for you, modifying some of its input or
           output to better suit your purposes.

           What You See Is What You Get.  Usually used when something that appears on the screen matches how it will
           eventually look, like Perl's format declarations.  Also used to mean the opposite of magic because every-
           thing works exactly as it appears, as in the three-argument form of open.


       XS  An extraordinarily exported, expeditiously excellent, expressly eXternal Subroutine, executed in existing C
           or C++ or in an exciting new extension language called (exasperatingly) XS.  Examine perlxs for the exact
           explanation or perlxstut for an exemplary unexacting one.

           An external "subroutine" defined in "XS".


           Yet Another Compiler Compiler.  A parser generator without which Perl probably would not have existed.  See
           the file perly.y in the Perl source distribution.


       zero width
           A subpattern "assertion" matching the "null string" between characters.

           A process that has died (exited) but whose parent has not yet received proper notification of its demise by
           virtue of having called wait or waitpid.  If you fork, you must clean up after your child processes when
           they exit, or else the process table will fill up and your system administrator will Not Be Happy with you.

       Based on the Glossary of Programming Perl, Third Edition, by Larry Wall, Tom Christiansen & Jon Orwant.  Copy-
       right (c) 2000, 1996, 1991 O'Reilly Media, Inc.  This document may be distributed under the same terms as Perl

perl v5.8.8                       2006-01-07                   PERLGLOSSARY(1)