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PERLRUN(1)             Perl Programmers Reference Guide             PERLRUN(1)

       perlrun - how to execute the Perl interpreter

       perl [ -sTtuUWX ]      [ -hv ] [ -V[:configvar] ]      [ -cw ] [ -d[t][:debugger] ] [ -D[number/list] ]
            [ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal/hexadecimal] ]      [ -Idir ] [ -m[-]module ] [ -M[-]'mod-
       ule...' ] [ -f ]      [ -C [number/list] ]      [ -P ]      [ -S ]      [ -x[dir] ]      [ -i[extension] ]
            [ -e 'command' ] [ -- ] [ programfile ] [ argument ]...

       The normal way to run a Perl program is by making it directly executable, or else by passing the name of the
       source file as an argument on the command line.  (An interactive Perl environment is also possible--see perlde-
       bug for details on how to do that.)  Upon startup, Perl looks for your program in one of the following places:

       1.  Specified line by line via -e switches on the command line.

       2.  Contained in the file specified by the first filename on the command line.  (Note that systems supporting
           the #! notation invoke interpreters this way. See "Location of Perl".)

       3.  Passed in implicitly via standard input.  This works only if there are no filename arguments--to pass argu-
           ments to a STDIN-read program you must explicitly specify a "-" for the program name.

       With methods 2 and 3, Perl starts parsing the input file from the beginning, unless you've specified a -x
       switch, in which case it scans for the first line starting with #! and containing the word "perl", and starts
       there instead.  This is useful for running a program embedded in a larger message.  (In this case you would
       indicate the end of the program using the "__END__" token.)

       The #! line is always examined for switches as the line is being parsed.  Thus, if you're on a machine that
       allows only one argument with the #! line, or worse, doesn't even recognize the #! line, you still can get con-
       sistent switch behavior regardless of how Perl was invoked, even if -x was used to find the beginning of the

       Because historically some operating systems silently chopped off kernel interpretation of the #! line after 32
       characters, some switches may be passed in on the command line, and some may not; you could even get a "-"
       without its letter, if you're not careful.  You probably want to make sure that all your switches fall either
       before or after that 32-character boundary.  Most switches don't actually care if they're processed redun-
       dantly, but getting a "-" instead of a complete switch could cause Perl to try to execute standard input
       instead of your program.  And a partial -I switch could also cause odd results.

       Some switches do care if they are processed twice, for instance combinations of -l and -0.  Either put all the
       switches after the 32-character boundary (if applicable), or replace the use of -0digits by "BEGIN{ $/ =
       "\0digits"; }".

       Parsing of the #! switches starts wherever "perl" is mentioned in the line.  The sequences "-*" and "- " are
       specifically ignored so that you could, if you were so inclined, say

           #!/bin/sh -- # -*- perl -*- -p
           eval 'exec perl -wS $0 ${1+"$@"}'
               if $running_under_some_shell;

       to let Perl see the -p switch.

       A similar trick involves the env program, if you have it.

           #!/usr/bin/env perl

       The examples above use a relative path to the perl interpreter, getting whatever version is first in the user's
       path.  If you want a specific version of Perl, say, perl5.005_57, you should place that directly in the #!
       line's path.

       If the #! line does not contain the word "perl", the program named after the #! is executed instead of the Perl
       interpreter.  This is slightly bizarre, but it helps people on machines that don't do #!, because they can tell
       a program that their SHELL is /usr/bin/perl, and Perl will then dispatch the program to the correct interpreter
       for them.

       After locating your program, Perl compiles the entire program to an internal form.  If there are any compila-
       tion errors, execution of the program is not attempted.  (This is unlike the typical shell script, which might
       run part-way through before finding a syntax error.)

       If the program is syntactically correct, it is executed.  If the program runs off the end without hitting an
       exit() or die() operator, an implicit exit(0) is provided to indicate successful completion.

       #! and quoting on non-Unix systems

       Unix's #! technique can be simulated on other systems:


               extproc perl -S -your_switches

           as the first line in "*.cmd" file (-S due to a bug in cmd.exe's 'extproc' handling).

           Create a batch file to run your program, and codify it in "ALTERNATE_SHEBANG" (see the dosish.h file in the
           source distribution for more information).

           The Win95/NT installation, when using the ActiveState installer for Perl, will modify the Registry to asso-
           ciate the .pl extension with the perl interpreter.  If you install Perl by other means (including building
           from the sources), you may have to modify the Registry yourself.  Note that this means you can no longer
           tell the difference between an executable Perl program and a Perl library file.

           Under "Classic" MacOS, a perl program will have the appropriate Creator and Type, so that double-clicking
           them will invoke the MacPerl application.  Under Mac OS X, clickable apps can be made from any "#!" script
           using Wil Sanchez' DropScript utility: .

       VMS Put

               $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
               $ exit++ + ++$status != 0 and $exit = $status = undef;

           at the top of your program, where -mysw are any command line switches you want to pass to Perl.  You can
           now invoke the program directly, by saying "perl program", or as a DCL procedure, by saying @program (or
           implicitly via DCL$PATH by just using the name of the program).

           This incantation is a bit much to remember, but Perl will display it for you if you say "perl "-V:start-

       Command-interpreters on non-Unix systems have rather different ideas on quoting than Unix shells.  You'll need
       to learn the special characters in your command-interpreter ("*", "\" and """ are common) and how to protect
       whitespace and these characters to run one-liners (see -e below).

       On some systems, you may have to change single-quotes to double ones, which you must not do on Unix or Plan 9
       systems.  You might also have to change a single % to a %%.

       For example:

           # Unix
           perl -e 'print "Hello world\n"'

           # MS-DOS, etc.
           perl -e "print \"Hello world\n\""

           # Macintosh
           print "Hello world\n"
            (then Run "Myscript" or Shift-Command-R)

           # VMS
           perl -e "print ""Hello world\n"""

       The problem is that none of this is reliable: it depends on the command and it is entirely possible neither
       works.  If 4DOS were the command shell, this would probably work better:

           perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""

       CMD.EXE in Windows NT slipped a lot of standard Unix functionality in when nobody was looking, but just try to
       find documentation for its quoting rules.

       Under the Macintosh, it depends which environment you are using.  The MacPerl shell, or MPW, is much like Unix
       shells in its support for several quoting variants, except that it makes free use of the Macintosh's non-ASCII
       characters as control characters.

       There is no general solution to all of this.  It's just a mess.

       Location of Perl

       It may seem obvious to say, but Perl is useful only when users can easily find it.  When possible, it's good
       for both /usr/bin/perl and /usr/local/bin/perl to be symlinks to the actual binary.  If that can't be done,
       system administrators are strongly encouraged to put (symlinks to) perl and its accompanying utilities into a
       directory typically found along a user's PATH, or in some other obvious and convenient place.

       In this documentation, "#!/usr/bin/perl" on the first line of the program will stand in for whatever method
       works on your system.  You are advised to use a specific path if you care about a specific version.


       or if you just want to be running at least version, place a statement like this at the top of your program:

           use 5.005_54;

       Command Switches

       As with all standard commands, a single-character switch may be clustered with the following switch, if any.

           #!/usr/bin/perl -spi.orig   # same as -s -p -i.orig

       Switches include:

            specifies the input record separator ($/) as an octal or hexadecimal number.  If there are no digits, the
            null character is the separator.  Other switches may precede or follow the digits.  For example, if you
            have a version of find which can print filenames terminated by the null character, you can say this:

                find . -name '*.orig' -print0 | perl -n0e unlink

            The special value 00 will cause Perl to slurp files in paragraph mode.  The value 0777 will cause Perl to
            slurp files whole because there is no legal byte with that value.

            If you want to specify any Unicode character, use the hexadecimal format: "-0xHHH...", where the "H" are
            valid hexadecimal digits.  (This means that you cannot use the "-x" with a directory name that consists of
            hexadecimal digits.)

       -a   turns on autosplit mode when used with a -n or -p.  An implicit split command to the @F array is done as
            the first thing inside the implicit while loop produced by the -n or -p.

                perl -ane 'print pop(@F), "\n";'

            is equivalent to

                while (<>) {
                    @F = split(' ');
                    print pop(@F), "\n";

            An alternate delimiter may be specified using -F.

       -C [number/list]
            The "-C" flag controls some Unicode of the Perl Unicode features.

            As of 5.8.1, the "-C" can be followed either by a number or a list of option letters.  The letters, their
            numeric values, and effects are as follows; listing the letters is equal to summing the numbers.

                I     1    STDIN is assumed to be in UTF-8
                O     2    STDOUT will be in UTF-8
                E     4    STDERR will be in UTF-8
                S     7    I + O + E
                i     8    UTF-8 is the default PerlIO layer for input streams
                o    16    UTF-8 is the default PerlIO layer for output streams
                D    24    i + o
                A    32    the @ARGV elements are expected to be strings encoded in UTF-8
                L    64    normally the "IOEioA" are unconditional,
                           the L makes them conditional on the locale environment
                           variables (the LC_ALL, LC_TYPE, and LANG, in the order
                           of decreasing precedence) -- if the variables indicate
                           UTF-8, then the selected "IOEioA" are in effect

            For example, "-COE" and "-C6" will both turn on UTF-8-ness on both STDOUT and STDERR.  Repeating letters
            is just redundant, not cumulative nor toggling.

            The "io" options mean that any subsequent open() (or similar I/O operations) will have the ":utf8" PerlIO
            layer implicitly applied to them, in other words, UTF-8 is expected from any input stream, and UTF-8 is
            produced to any output stream.  This is just the default, with explicit layers in open() and with bin-
            mode() one can manipulate streams as usual.

            "-C" on its own (not followed by any number or option list), or the empty string "" for the "PERL_UNICODE"
            environment variable, has the same effect as "-CSDL".  In other words, the standard I/O handles and the
            default "open()" layer are UTF-8-fied but only if the locale environment variables indicate a UTF-8
            locale.  This behaviour follows the implicit (and problematic) UTF-8 behaviour of Perl 5.8.0.

            You can use "-C0" (or "0" for "PERL_UNICODE") to explicitly disable all the above Unicode features.

            The read-only magic variable "${^UNICODE}" reflects the numeric value of this setting.  This is variable
            is set during Perl startup and is thereafter read-only.  If you want runtime effects, use the three-arg
            open() (see "open" in perlfunc), the two-arg binmode() (see "binmode" in perlfunc), and the "open" pragma
            (see open).

            (In Perls earlier than 5.8.1 the "-C" switch was a Win32-only switch that enabled the use of Unicode-aware
            "wide system call" Win32 APIs.  This feature was practically unused, however, and the command line switch
            was therefore "recycled".)

       -c   causes Perl to check the syntax of the program and then exit without executing it.  Actually, it will exe-
            cute "BEGIN", "CHECK", and "use" blocks, because these are considered as occurring outside the execution
            of your program.  "INIT" and "END" blocks, however, will be skipped.

       -dt  runs the program under the Perl debugger.  See perldebug.  If t is specified, it indicates to the debugger
            that threads will be used in the code being debugged.

            runs the program under the control of a debugging, profiling, or tracing module installed as Devel::foo.
            E.g., -d:DProf executes the program using the Devel::DProf profiler.  As with the -M flag, options may be
            passed to the Devel::foo package where they will be received and interpreted by the Devel::foo::import
            routine.  The comma-separated list of options must follow a "=" character.  If t is specified, it indi-
            cates to the debugger that threads will be used in the code being debugged.  See perldebug.

            sets debugging flags.  To watch how it executes your program, use -Dtls.  (This works only if debugging is
            compiled into your Perl.)  Another nice value is -Dx, which lists your compiled syntax tree.  And -Dr
            displays compiled regular expressions; the format of the output is explained in perldebguts.

            As an alternative, specify a number instead of list of letters (e.g., -D14 is equivalent to -Dtls):

                    1  p  Tokenizing and parsing
                    2  s  Stack snapshots (with v, displays all stacks)
                    4  l  Context (loop) stack processing
                    8  t  Trace execution
                   16  o  Method and overloading resolution
                   32  c  String/numeric conversions
                   64  P  Print profiling info, preprocessor command for -P, source file input state
                  128  m  Memory allocation
                  256  f  Format processing
                  512  r  Regular expression parsing and execution
                 1024  x  Syntax tree dump
                 2048  u  Tainting checks
                 4096     (Obsolete, previously used for LEAKTEST)
                 8192  H  Hash dump -- usurps values()
                16384  X  Scratchpad allocation
                32768  D  Cleaning up
                65536  S  Thread synchronization
               131072  T  Tokenising
               262144  R  Include reference counts of dumped variables (eg when using -Ds)
               524288  J  Do not s,t,P-debug (Jump over) opcodes within package DB
              1048576  v  Verbose: use in conjunction with other flags
              8388608  q  quiet - currently only suppresses the "EXECUTING" message

            All these flags require -DDEBUGGING when you compile the Perl executable (but see Devel::Peek, re which
            may change this).  See the INSTALL file in the Perl source distribution for how to do this.  This flag is
            automatically set if you include -g option when "Configure" asks you about optimizer/debugger flags.

            If you're just trying to get a print out of each line of Perl code as it executes, the way that "sh -x"
            provides for shell scripts, you can't use Perl's -D switch.  Instead do this

              # If you have "env" utility
              env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program

              # Bourne shell syntax
              $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program

              # csh syntax
              % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)

            See perldebug for details and variations.

       -e commandline
            may be used to enter one line of program.  If -e is given, Perl will not look for a filename in the argu-
            ment list.  Multiple -e commands may be given to build up a multi-line script.  Make sure to use semi-
            colons where you would in a normal program.

       -f   Disable executing $Config{sitelib}/ at startup.

            Perl can be built so that it by default will try to execute $Config{sitelib}/ at startup.
            This is a hook that allows the sysadmin to customize how perl behaves.  It can for instance be used to add
            entries to the @INC array to make perl find modules in non-standard locations.

            specifies the pattern to split on if -a is also in effect.  The pattern may be surrounded by "//", "", or
            '', otherwise it will be put in single quotes. You can't use literal whitespace in the pattern.

       -h   prints a summary of the options.

            specifies that files processed by the "<>" construct are to be edited in-place.  It does this by renaming
            the input file, opening the output file by the original name, and selecting that output file as the
            default for print() statements.  The extension, if supplied, is used to modify the name of the old file to
            make a backup copy, following these rules:

            If no extension is supplied, no backup is made and the current file is overwritten.

            If the extension doesn't contain a "*", then it is appended to the end of the current filename as a suf-
            fix.  If the extension does contain one or more "*" characters, then each "*" is replaced with the current
            filename.  In Perl terms, you could think of this as:

                ($backup = $extension) =~ s/\*/$file_name/g;

            This allows you to add a prefix to the backup file, instead of (or in addition to) a suffix:

                $ perl -pi'orig_*' -e 's/bar/baz/' fileA    # backup to 'orig_fileA'

            Or even to place backup copies of the original files into another directory (provided the directory
            already exists):

                $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'

            These sets of one-liners are equivalent:

                $ perl -pi -e 's/bar/baz/' fileA            # overwrite current file
                $ perl -pi'*' -e 's/bar/baz/' fileA         # overwrite current file

                $ perl -pi'.orig' -e 's/bar/baz/' fileA     # backup to 'fileA.orig'
                $ perl -pi'*.orig' -e 's/bar/baz/' fileA    # backup to 'fileA.orig'

            From the shell, saying

                $ perl -p -i.orig -e "s/foo/bar/; ... "

            is the same as using the program:

                #!/usr/bin/perl -pi.orig

            which is equivalent to

                $extension = '.orig';
                LINE: while (<>) {
                    if ($ARGV ne $oldargv) {
                        if ($extension !~ /\*/) {
                            $backup = $ARGV . $extension;
                        else {
                            ($backup = $extension) =~ s/\*/$ARGV/g;
                        rename($ARGV, $backup);
                        open(ARGVOUT, ">$ARGV");
                        $oldargv = $ARGV;
                continue {
                    print;  # this prints to original filename

            except that the -i form doesn't need to compare $ARGV to $oldargv to know when the filename has changed.
            It does, however, use ARGVOUT for the selected filehandle.  Note that STDOUT is restored as the default
            output filehandle after the loop.

            As shown above, Perl creates the backup file whether or not any output is actually changed.  So this is
            just a fancy way to copy files:

                $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
                $ perl -p -i'.orig' -e 1 file1 file2 file3...

            You can use "eof" without parentheses to locate the end of each input file, in case you want to append to
            each file, or reset line numbering (see example in "eof" in perlfunc).

            If, for a given file, Perl is unable to create the backup file as specified in the extension then it will
            skip that file and continue on with the next one (if it exists).

            For a discussion of issues surrounding file permissions and -i, see "Why does Perl let me delete read-only
            files? Why does -i clobber protected files? Isn't this a bug in Perl?" in perlfaq5.

            You cannot use -i to create directories or to strip extensions from files.

            Perl does not expand "~" in filenames, which is good, since some folks use it for their backup files:

                $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...

            Note that because -i renames or deletes the original file before creating a new file of the same name,
            UNIX-style soft and hard links will not be preserved.

            Finally, the -i switch does not impede execution when no files are given on the command line.  In this
            case, no backup is made (the original file cannot, of course, be determined) and processing proceeds from
            STDIN to STDOUT as might be expected.

            Directories specified by -I are prepended to the search path for modules (@INC), and also tells the C pre-
            processor where to search for include files.  The C preprocessor is invoked with -P; by default it
            searches /usr/include and /usr/lib/perl.

            enables automatic line-ending processing.  It has two separate effects.  First, it automatically chomps $/
            (the input record separator) when used with -n or -p.  Second, it assigns "$\" (the output record separa-
            tor) to have the value of octnum so that any print statements will have that separator added back on.  If
            octnum is omitted, sets "$\" to the current value of $/.  For instance, to trim lines to 80 columns:

                perl -lpe 'substr($_, 80) = ""'

            Note that the assignment "$\ = $/" is done when the switch is processed, so the input record separator can
            be different than the output record separator if the -l switch is followed by a -0 switch:

                gnufind / -print0 | perl -ln0e 'print "found $_" if -p'

            This sets "$\" to newline and then sets $/ to the null character.

       -M[-]'module ...'
            -mmodule executes "use" module "();" before executing your program.

            -Mmodule executes "use" module ";" before executing your program.  You can use quotes to add extra code
            after the module name, e.g., '-Mmodule qw(foo bar)'.

            If the first character after the -M or -m is a dash ("-") then the 'use' is replaced with 'no'.

            A little builtin syntactic sugar means you can also say -mmodule=foo,bar or -Mmodule=foo,bar as a shortcut
            for '-Mmodule qw(foo bar)'.  This avoids the need to use quotes when importing symbols.  The actual code
            generated by -Mmodule=foo,bar is "use module split(/,/,q{foo,bar})".  Note that the "=" form removes the
            distinction between -m and -M.

            A consequence of this is that -MFoo=number never does a version check (unless "Foo::import()" itself is
            set up to do a version check, which could happen for example if Foo inherits from Exporter.)

       -n   causes Perl to assume the following loop around your program, which makes it iterate over filename argu-
            ments somewhat like sed -n or awk:

                while (<>) {
                    ...             # your program goes here

            Note that the lines are not printed by default.  See -p to have lines printed.  If a file named by an
            argument cannot be opened for some reason, Perl warns you about it and moves on to the next file.

            Here is an efficient way to delete all files that haven't been modified for at least a week:

                find . -mtime +7 -print | perl -nle unlink

            This is faster than using the -exec switch of find because you don't have to start a process on every
            filename found.  It does suffer from the bug of mishandling newlines in pathnames, which you can fix if
            you follow the example under -0.

            "BEGIN" and "END" blocks may be used to capture control before or after the implicit program loop, just as
            in awk.

       -p   causes Perl to assume the following loop around your program, which makes it iterate over filename argu-
            ments somewhat like sed:

                while (<>) {
                    ...             # your program goes here
                } continue {
                    print or die "-p destination: $!\n";

            If a file named by an argument cannot be opened for some reason, Perl warns you about it, and moves on to
            the next file.  Note that the lines are printed automatically.  An error occurring during printing is
            treated as fatal.  To suppress printing use the -n switch.  A -p overrides a -n switch.

            "BEGIN" and "END" blocks may be used to capture control before or after the implicit loop, just as in awk.

       -P   NOTE: Use of -P is strongly discouraged because of its inherent problems, including poor portability.

            This option causes your program to be run through the C preprocessor before compilation by Perl.  Because
            both comments and cpp directives begin with the # character, you should avoid starting comments with any
            words recognized by the C preprocessor such as "if", "else", or "define".

            If you're considering using "-P", you might also want to look at the Filter::cpp module from CPAN.

            The problems of -P include, but are not limited to:

            *         The "#!" line is stripped, so any switches there don't apply.

            *         A "-P" on a "#!" line doesn't work.

            *         All lines that begin with (whitespace and) a "#" but do not look like cpp commands, are
                      stripped, including anything inside Perl strings, regular expressions, and here-docs .

            *         In some platforms the C preprocessor knows too much: it knows about the C++ -style until-end-of-
                      line comments starting with "//".  This will cause problems with common Perl constructs like


                      because after -P this will became illegal code


                      The workaround is to use some other quoting separator than "/", like for example "!":


            *         It requires not only a working C preprocessor but also a working sed.  If not on UNIX, you are
                      probably out of luck on this.

            *         Script line numbers are not preserved.

            *         The "-x" does not work with "-P".

       -s   enables rudimentary switch parsing for switches on the command line after the program name but before any
            filename arguments (or before an argument of --).  Any switch found there is removed from @ARGV and sets
            the corresponding variable in the Perl program.  The following program prints "1" if the program is
            invoked with a -xyz switch, and "abc" if it is invoked with -xyz=abc.

                #!/usr/bin/perl -s
                if ($xyz) { print "$xyz\n" }

            Do note that a switch like --help creates the variable ${-help}, which is not compliant with "strict
            refs".  Also, when using this option on a script with warnings enabled you may get a lot of spurious "used
            only once" warnings.

       -S   makes Perl use the PATH environment variable to search for the program (unless the name of the program
            contains directory separators).

            On some platforms, this also makes Perl append suffixes to the filename while searching for it.  For exam-
            ple, on Win32 platforms, the ".bat" and ".cmd" suffixes are appended if a lookup for the original name
            fails, and if the name does not already end in one of those suffixes.  If your Perl was compiled with
            DEBUGGING turned on, using the -Dp switch to Perl shows how the search progresses.

            Typically this is used to emulate #! startup on platforms that don't support #!.  Its also convenient when
            debugging a script that uses #!, and is thus normally found by the shell's $PATH search mechanism.

            This example works on many platforms that have a shell compatible with Bourne shell:

                eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
                        if $running_under_some_shell;

            The system ignores the first line and feeds the program to /bin/sh, which proceeds to try to execute the
            Perl program as a shell script.  The shell executes the second line as a normal shell command, and thus
            starts up the Perl interpreter.  On some systems $0 doesn't always contain the full pathname, so the -S
            tells Perl to search for the program if necessary.  After Perl locates the program, it parses the lines
            and ignores them because the variable $running_under_some_shell is never true.  If the program will be
            interpreted by csh, you will need to replace "${1+"$@"}" with $*, even though that doesn't understand
            embedded spaces (and such) in the argument list.  To start up sh rather than csh, some systems may have to
            replace the #! line with a line containing just a colon, which will be politely ignored by Perl.  Other
            systems can't control that, and need a totally devious construct that will work under any of csh, sh, or
            Perl, such as the following:

                    eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
                    & eval 'exec /usr/bin/perl -wS $0 $argv:q'
                            if $running_under_some_shell;

            If the filename supplied contains directory separators (i.e., is an absolute or relative pathname), and if
            that file is not found, platforms that append file extensions will do so and try to look for the file with
            those extensions added, one by one.

            On DOS-like platforms, if the program does not contain directory separators, it will first be searched for
            in the current directory before being searched for on the PATH.  On Unix platforms, the program will be
            searched for strictly on the PATH.

       -t   Like -T, but taint checks will issue warnings rather than fatal errors.  These warnings can be controlled
            normally with "no warnings qw(taint)".

            NOTE: this is not a substitute for -T. This is meant only to be used as a temporary development aid while
            securing legacy code: for real production code and for new secure code written from scratch always use the
            real -T.

       -T   forces "taint" checks to be turned on so you can test them.  Ordinarily these checks are done only when
            running setuid or setgid.  It's a good idea to turn them on explicitly for programs that run on behalf of
            someone else whom you might not necessarily trust, such as CGI programs or any internet servers you might
            write in Perl.  See perlsec for details.  For security reasons, this option must be seen by Perl quite
            early; usually this means it must appear early on the command line or in the #! line for systems which
            support that construct.

       -u   This obsolete switch causes Perl to dump core after compiling your program.  You can then in theory take
            this core dump and turn it into an executable file by using the undump program (not supplied).  This
            speeds startup at the expense of some disk space (which you can minimize by stripping the executable).
            (Still, a "hello world" executable comes out to about 200K on my machine.)  If you want to execute a por-
            tion of your program before dumping, use the dump() operator instead.  Note: availability of undump is
            platform specific and may not be available for a specific port of Perl.

            This switch has been superseded in favor of the new Perl code generator backends to the compiler.  See B
            and B::Bytecode for details.

       -U   allows Perl to do unsafe operations.  Currently the only "unsafe" operations are attempting to unlink
            directories while running as superuser, and running setuid programs with fatal taint checks turned into
            warnings.  Note that the -w switch (or the $^W variable) must be used along with this option to actually
            generate the taint-check warnings.

       -v   prints the version and patchlevel of your perl executable.

       -V   prints summary of the major perl configuration values and the current values of @INC.

            Prints to STDOUT the value of the named configuration variable(s), with multiples when your configvar
            argument looks like a regex (has non-letters).  For example:

                $ perl -V:libc
                $ perl -V:lib.
                    libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
                $ perl -V:lib.*
                    libpth='/usr/local/lib /lib /usr/lib';
                    libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';

            Additionally, extra colons can be used to control formatting.  A trailing colon suppresses the linefeed
            and terminator ';', allowing you to embed queries into shell commands.  (mnemonic: PATH separator ':'.)

                $ echo "compression-vars: " 'perl -V:z.*: ' " are here !"
                compression-vars:  zcat='' zip='zip'  are here !

            A leading colon removes the 'name=' part of the response, this allows you to map to the name you need.
            (mnemonic: empty label)

                $ echo "goodvfork="'./perl -Ilib -V::usevfork'

            Leading and trailing colons can be used together if you need positional parameter values without the
            names.  Note that in the case below, the PERL_API params are returned in alphabetical order.

                $ echo building_on 'perl -V::osname: -V::PERL_API_.*:' now
                building_on 'linux' '5' '1' '9' now

       -w   prints warnings about dubious constructs, such as variable names that are mentioned only once and scalar
            variables that are used before being set, redefined subroutines, references to undefined filehandles or
            filehandles opened read-only that you are attempting to write on, values used as a number that don't look
            like numbers, using an array as though it were a scalar, if your subroutines recurse more than 100 deep,
            and innumerable other things.

            This switch really just enables the internal $^W variable.  You can disable or promote into fatal errors
            specific warnings using "__WARN__" hooks, as described in perlvar and "warn" in perlfunc.  See also perl-
            diag and perltrap.  A new, fine-grained warning facility is also available if you want to manipulate
            entire classes of warnings; see warnings or perllexwarn.

       -W   Enables all warnings regardless of "no warnings" or $^W.  See perllexwarn.

       -X   Disables all warnings regardless of "use warnings" or $^W.  See perllexwarn.

       -x directory
            tells Perl that the program is embedded in a larger chunk of unrelated ASCII text, such as in a mail mes-
            sage.  Leading garbage will be discarded until the first line that starts with #! and contains the string
            "perl".  Any meaningful switches on that line will be applied.  If a directory name is specified, Perl
            will switch to that directory before running the program.  The -x switch controls only the disposal of
            leading garbage.  The program must be terminated with "__END__" if there is trailing garbage to be ignored
            (the program can process any or all of the trailing garbage via the DATA filehandle if desired).

       HOME        Used if chdir has no argument.

       LOGDIR      Used if chdir has no argument and HOME is not set.

       PATH        Used in executing subprocesses, and in finding the program if -S is used.

       PERL5LIB    A list of directories in which to look for Perl library files before looking in the standard
                   library and the current directory.  Any architecture-specific directories under the specified loca-
                   tions are automatically included if they exist.  If PERL5LIB is not defined, PERLLIB is used.
                   Directories are separated (like in PATH) by a colon on unixish platforms and by a semicolon on Win-
                   dows (the proper path separator being given by the command "perl -V:path_sep").

                   When running taint checks (either because the program was running setuid or setgid, or the -T
                   switch was used), neither variable is used.  The program should instead say:

                       use lib "/my/directory";

       PERL5OPT    Command-line options (switches).  Switches in this variable are taken as if they were on every Perl
                   command line.  Only the -[DIMUdmtw] switches are allowed.  When running taint checks (because the
                   program was running setuid or setgid, or the -T switch was used), this variable is ignored.  If
                   PERL5OPT begins with -T, tainting will be enabled, and any subsequent options ignored.

       PERLIO      A space (or colon) separated list of PerlIO layers. If perl is built to use PerlIO system for IO
                   (the default) these layers effect perl's IO.

                   It is conventional to start layer names with a colon e.g. ":perlio" to emphasise their similarity
                   to variable "attributes". But the code that parses layer specification strings (which is also used
                   to decode the PERLIO environment variable) treats the colon as a separator.

                   An unset or empty PERLIO is equivalent to ":stdio".

                   The list becomes the default for all perl's IO. Consequently only built-in layers can appear in
                   this list, as external layers (such as :encoding()) need IO in  order to load them!. See "open
                   pragma" for how to add external encodings as defaults.

                   The layers that it makes sense to include in the PERLIO environment variable are briefly summarised
                   below. For more details see PerlIO.

                   :bytes  A pseudolayer that turns off the ":utf8" flag for the layer below.  Unlikely to be useful
                           on its own in the global PERLIO environment variable.  You perhaps were thinking of
                           ":crlf:bytes" or ":perlio:bytes".

                   :crlf   A layer which does CRLF to "\n" translation distinguishing "text" and "binary" files in the
                           manner of MS-DOS and similar operating systems.  (It currently does not mimic MS-DOS as far
                           as treating of Control-Z as being an end-of-file marker.)

                   :mmap   A layer which implements "reading" of files by using "mmap()" to make (whole) file appear
                           in the process's address space, and then using that as PerlIO's "buffer".

                   :perlio This is a re-implementation of "stdio-like" buffering written as a PerlIO "layer".  As such
                           it will call whatever layer is below it for its operations (typically ":unix").

                   :pop    An experimental pseudolayer that removes the topmost layer.  Use with the same care as is
                           reserved for nitroglycerin.

                   :raw    A pseudolayer that manipulates other layers.  Applying the ":raw" layer is equivalent to
                           calling "binmode($fh)".  It makes the stream pass each byte as-is without any translation.
                           In particular CRLF translation, and/or :utf8 intuited from locale are disabled.

                           Unlike in the earlier versions of Perl ":raw" is not just the inverse of ":crlf" - other
                           layers which would affect the binary nature of the stream are also removed or disabled.

                   :stdio  This layer provides PerlIO interface by wrapping system's ANSI C "stdio" library calls. The
                           layer provides both buffering and IO.  Note that ":stdio" layer does not do CRLF
                           translation even if that is platforms normal behaviour. You will need a ":crlf" layer above
                           it to do that.

                   :unix   Low level layer which calls "read", "write" and "lseek" etc.

                   :utf8   A pseudolayer that turns on a flag on the layer below to tell perl that output should be in
                           utf8 and that input should be regarded as already in utf8 form.  May be useful in PERLIO
                           environment variable to make UTF-8 the default. (To turn off that behaviour use ":bytes"

                   :win32  On Win32 platforms this experimental layer uses native "handle" IO rather than unix-like
                           numeric file descriptor layer. Known to be buggy in this release.

                   On all platforms the default set of layers should give acceptable results.

                   For UNIX platforms that will equivalent of "unix perlio" or "stdio".  Configure is setup to prefer
                   "stdio" implementation if system's library provides for fast access to the buffer, otherwise it
                   uses the "unix perlio" implementation.

                   On Win32 the default in this release is "unix crlf". Win32's "stdio" has a number of bugs/mis-fea-
                   tures for perl IO which are somewhat C compiler vendor/version dependent. Using our own "crlf"
                   layer as the buffer avoids those issues and makes things more uniform.  The "crlf" layer provides
                   CRLF to/from "\n" conversion as well as buffering.

                   This release uses "unix" as the bottom layer on Win32 and so still uses C compiler's numeric file
                   descriptor routines. There is an experimental native "win32" layer which is expected to be enhanced
                   and should eventually be the default under Win32.

                   If set to the name of a file or device then certain operations of PerlIO sub-system will be logged
                   to that file (opened as append). Typical uses are UNIX:

                      PERLIO_DEBUG=/dev/tty perl script ...

                   and Win32 approximate equivalent:

                      set PERLIO_DEBUG=CON
                      perl script ...

                   This functionality is disabled for setuid scripts and for scripts run with -T.

       PERLLIB     A list of directories in which to look for Perl library files before looking in the standard
                   library and the current directory.  If PERL5LIB is defined, PERLLIB is not used.

       PERL5DB     The command used to load the debugger code.  The default is:

                           BEGIN { require '' }

                   If set to a true value, indicates to the debugger that the code being debugged uses threads.

       PERL5SHELL (specific to the Win32 port)
                   May be set to an alternative shell that perl must use internally for executing "backtick" commands
                   or system().  Default is "cmd.exe /x/d/c" on WindowsNT and " /c" on Windows95.  The
                   value is considered to be space-separated.  Precede any character that needs to be protected (like
                   a space or backslash) with a backslash.

                   Note that Perl doesn't use COMSPEC for this purpose because COMSPEC has a high degree of variabil-
                   ity among users, leading to portability concerns.  Besides, perl can use a shell that may not be
                   fit for interactive use, and setting COMSPEC to such a shell may interfere with the proper func-
                   tioning of other programs (which usually look in COMSPEC to find a shell fit for interactive use).

       PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
                   Set to 1 to allow the use of non-IFS compatible LSP's.  Perl normally searches for an IFS-compati-
                   ble LSP because this is required for its emulation of Windows sockets as real filehandles.  How-
                   ever, this may cause problems if you have a firewall such as McAfee Guardian which requires all
                   applications to use its LSP which is not IFS-compatible, because clearly Perl will normally avoid
                   using such an LSP.  Setting this environment variable to 1 means that Perl will simply use the
                   first suitable LSP enumerated in the catalog, which keeps McAfee Guardian happy (and in that par-
                   ticular case Perl still works too because McAfee Guardian's LSP actually plays some other games
                   which allow applications requiring IFS compatibility to work).

                   Relevant only if perl is compiled with the malloc included with the perl distribution (that is, if
                   "perl -V:d_mymalloc" is 'define').  If set, this causes memory statistics to be dumped after execu-
                   tion.  If set to an integer greater than one, also causes memory statistics to be dumped after com-

                   Relevant only if your perl executable was built with -DDEBUGGING, this controls the behavior of
                   global destruction of objects and other references.  See "PERL_DESTRUCT_LEVEL" in perlhack for more

                   Set to one to have perl resolve all undefined symbols when it loads a dynamic library.  The default
                   behaviour is to resolve symbols when they are used.  Setting this variable is useful during testing
                   of extensions as it ensures that you get an error on misspelled function names even if the test
                   suite doesn't call it.

                   If using the "encoding" pragma without an explicit encoding name, the PERL_ENCODING environment
                   variable is consulted for an encoding name.

                   (Since Perl 5.8.1.)  Used to randomise Perl's internal hash function.  To emulate the pre-5.8.1
                   behaviour, set to an integer (zero means exactly the same order as 5.8.0).  "Pre-5.8.1" means,
                   among other things, that hash keys will be ordered the same between different runs of Perl.

                   The default behaviour is to randomise unless the PERL_HASH_SEED is set.  If Perl has been compiled
                   with "-DUSE_HASH_SEED_EXPLICIT", the default behaviour is not to randomise unless the
                   PERL_HASH_SEED is set.

                   If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses the pseudorandom seed supplied
                   by the operating system and libraries.  This means that each different run of Perl will have a dif-
                   ferent ordering of the results of keys(), values(), and each().

                   Please note that the hash seed is sensitive information. Hashes are randomized to protect against
                   local and remote attacks against Perl code. By manually setting a seed this protection may be par-
                   tially or completely lost.

                   See "Algorithmic Complexity Attacks" in perlsec and "PERL_HASH_SEED_DEBUG" for more information.

                   (Since Perl 5.8.1.)  Set to one to display (to STDERR) the value of the hash seed at the beginning
                   of execution.  This, combined with "PERL_HASH_SEED" is intended to aid in debugging nondeterminis-
                   tic behavior caused by hash randomization.

                   Note that the hash seed is sensitive information: by knowing it one can craft a denial-of-service
                   attack against Perl code, even remotely, see "Algorithmic Complexity Attacks" in perlsec for more
                   information.  Do not disclose the hash seed to people who don't need to know it.  See also
                   hash_seed() of Hash::Util.

       PERL_ROOT (specific to the VMS port)
                   A translation concealed rooted logical name that contains perl and the logical device for the @INC
                   path on VMS only.  Other logical names that affect perl on VMS include PERLSHR, PERL_ENV_TABLES,
                   and SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in perlvms and in README.vms
                   in the Perl source distribution.

                   In Perls 5.8.1 and later.  If set to "unsafe" the pre-Perl-5.8.0 signals behaviour (immediate but
                   unsafe) is restored.  If set to "safe" the safe (or deferred) signals are used.  See "Deferred Sig-
                   nals (Safe Signals)" in perlipc.

                   Equivalent to the -C command-line switch.  Note that this is not a boolean variable-- setting this
                   to "1" is not the right way to "enable Unicode" (whatever that would mean).  You can use "0" to
                   "disable Unicode", though (or alternatively unset PERL_UNICODE in your shell before starting Perl).
                   See the description of the "-C" switch for more information.

       SYS$LOGIN (specific to the VMS port)
                   Used if chdir has no argument and HOME and LOGDIR are not set.

       Perl also has environment variables that control how Perl handles data specific to particular natural lan-
       guages.  See perllocale.

       Apart from these, Perl uses no other environment variables, except to make them available to the program being
       executed, and to child processes.  However, programs running setuid would do well to execute the following
       lines before doing anything else, just to keep people honest:

           $ENV{PATH}  = '/bin:/usr/bin';    # or whatever you need
           $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
           delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};

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