perlrun - how to execute the Perl interpreter


   perl [-sTtuUWX]      [-hv][-V[:configvar]]
        [-C[number/list]]      [-S]      [-x[dir]]


   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 perldebug 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 or -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 arguments 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 consistent switch behaviour regardless of how Perl was
   invoked, even if -x was used to find the beginning of the program.

   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 redundantly, 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

       #! -*-perl-*-
       eval 'exec perl -x -wS $0 ${1+"$@"}'
           if 0;

   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, perl5.14.1, you should place that directly in the
   "#!" line's path.

   If the "#!" line does not contain the word "perl" nor the word "indir",
   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 compilation 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 associate 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

   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:startperl"".

   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

   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\""

       # 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.

   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

   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


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

       use 5.014;

   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

   A "--" signals the end of options and disables further option
   processing. Any arguments after the "--" are treated as filenames and

   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.  Any value 0400 or above will cause Perl to slurp files
        whole, but by convention the value 0777 is the one normally used
        for this purpose.

        You can also specify the separator character using hexadecimal
        notation: -0xHHH..., where the "H" are valid hexadecimal digits.
        Unlike the octal form, this one may be used to specify any Unicode
        character, even those beyond 0xFF.  So if you really want a record
        separator of 0777, specify it as -0x1FF.  (This means that you
        cannot use the -x option with a directory name that consists of
        hexadecimal digits, or else Perl will think you have specified a
        hex number to -0.)

   -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.

        -a implicitly sets -n.

   -C [number/list]
        The -C flag controls some 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

            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_CTYPE, and LANG, in the order of
                      decreasing precedence) -- if the variables indicate
                      UTF-8, then the selected "IOEioA" are in effect
            a   256   Set ${^UTF8CACHE} to -1, to run the UTF-8 caching
                      code in debugging mode.

        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) in the current file scope 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 binmode() 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.  (See "UTF-8 no longer default under UTF-8 locales"
        in perl581delta.)

        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 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".)

        Note: Since perl 5.10.1, if the -C option is used on the "#!"
        line, it must be specified on the command line as well, since the
        standard streams are already set up at this point in the execution
        of the perl interpreter.  You can also use binmode() to set the
        encoding of an I/O stream.

   -c   causes Perl to check the syntax of the program and then exit
        without executing it.  Actually, it will execute and "BEGIN",
        "UNITCHECK", or "CHECK" blocks and any "use" statements: 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::MOD". E.g., -d:DProf executes
        the program using the "Devel::DProf" profiler.  As with the -M
        flag, options may be passed to the "Devel::MOD" package where they
        will be received and interpreted by the "Devel::MOD::import"
        routine.  Again, like -M, use --d:-MOD to call
        "Devel::MOD::unimport" instead of import.  The comma-separated
        list of options must follow a "=" character.  If t is specified,
        it indicates 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 (with v, displays parse
                 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, source file input state
               128  m  Memory and SV allocation
               256  f  Format processing
               512  r  Regular expression parsing and execution
              1024  x  Syntax tree dump
              2048  u  Tainting checks
              4096  U  Unofficial, User hacking (reserved for private,
                       unreleased use)
              8192  H  Hash dump -- usurps values()
             16384  X  Scratchpad allocation
             32768  D  Cleaning up
             65536  S  Op slab allocation
            131072  T  Tokenizing
            262144  R  Include reference counts of dumped variables
                       (eg when using -Ds)
            524288  J  show s,t,P-debug (don't Jump over) on opcodes within
                       package DB
           1048576  v  Verbose: use in conjunction with other flags
           2097152  C  Copy On Write
           4194304  A  Consistency checks on internal structures
           8388608  q  quiet - currently only suppresses the "EXECUTING"
          16777216  M  trace smart match resolution
          33554432  B  dump suBroutine definitions, including special
                       Blocks like BEGIN
          67108864  L  trace Locale-related info; what gets output is very
                       subject to change
         134217728  i  trace PerlIO layer processing.  Set PERLIO_DEBUG to
                       the filename to trace to.

        All these flags require -DDEBUGGING when you compile the Perl
        executable (but see ":opd" in Devel::Peek or "'debug' mode" in 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 argument list.  Multiple -e
        commands may be given to build up a multi-line script.  Make sure
        to use semicolons where you would in a normal program.

   -E commandline
        behaves just like -e, except that it implicitly enables all
        optional features (in the main compilation unit). See feature.

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

        Perl can be built so that it by default will try to execute
        $Config{sitelib}/ at startup (in a BEGIN block).
        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.

        Perl actually inserts the following code:

            BEGIN {
                do { local $!; -f "$Config{sitelib}/"; }
                    && do "$Config{sitelib}/";

        Since it is an actual "do" (not a "require"),
        doesn't need to return a true value. The code is run in package
        "main", in its own lexical scope. However, if the script dies, $@
        will not be set.

        The value of $Config{sitelib} is also determined in C code and not
        read from "", which is not loaded.

        The code is executed very early. For example, any changes made to
        @INC will show up in the output of `perl -V`. Of course, "END"
        blocks will be likewise executed very late.

        To determine at runtime if this capability has been compiled in
        your perl, you can check the value of $Config{usesitecustomize}.

        Note: on Debian based systems, the system perl currently uses
        "/etc/perl/" rather than
        "$Config{sitelib}/".  This may change in the
        future and is only provided as a temporary transition mechanism
        for removing the current working directory from @INC.

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

        -F implicitly sets both -a and -n.

   -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, and your system supports it, the
        original file is kept open without a name while the output is
        redirected to a new file with the original filename.  When perl
        exits, cleanly or not, the original file is unlinked.

        If the extension doesn't contain a "*", then it is appended to the
        end of the current filename as a suffix.  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

            $ 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).

        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 separator) 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

            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

        -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; that is, -mMODULE=foo,bar is the
        same as -MMODULE=foo,bar.

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

   -n   causes Perl to assume the following loop around your program,
        which makes it iterate over filename arguments 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

        Also note that "<>" passes command line arguments to "open" in
        perlfunc, which doesn't necessarily interpret them as file names.
        See  perlop for possible security implications.

        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 (but it's
        not faster than using the -delete switch available in newer
        versions of find.  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 arguments 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.

   -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 "use 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 path separators.

        On some platforms, this also makes Perl append suffixes to the
        filename while searching for it.  For example, 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

        Typically this is used to emulate "#!" startup on platforms that
        don't support "#!".  It's 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 (and so 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 now be controlled normally with "no
        warnings qw(taint)".

        Note: This is not a substitute for "-T"! This is meant to be used
        only 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   turns on "taint" 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

   -u   This 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 portion 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.

   -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 warnings must be enabled 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

            $ 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
        mentioned only once and scalar variables used before being set;
        redefined subroutines; references to undefined filehandles;
        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 global $^W variable; normally,
        the lexically scoped "use warnings" pragma is preferred. You can
        disable or promote into fatal errors specific warnings using
        "__WARN__" hooks, as described in perlvar and "warn" in perlfunc.
        See also perldiag and perltrap.  A fine-grained warning facility
        is also available if you want to manipulate entire classes of
        warnings; see warnings.

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

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

        tells Perl that the program is embedded in a larger chunk of
        unrelated text, such as in a mail message.  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.

        All references to line numbers by the program (warnings, errors,
        ...)  will treat the "#!" line as the first line.  Thus a warning
        on the 2nd line of the program, which is on the 100th line in the
        file will be reported as line 2, not as line 100.  This can be
        overridden by using the "#line" directive.  (See "Plain Old
        Comments (Not!)" in perlsyn)

        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.

        The directory, if specified, must appear immediately following the
        -x with no intervening whitespace.


   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 and version-
               specific directories, such as version/archname/, version/,
               or archname/ under the specified locations are
               automatically included if they exist, with this lookup done
               at interpreter startup time.  In addition, any directories
               matching the entries in $Config{inc_version_list} are
               added.  (These typically would be for older compatible perl
               versions installed in the same directory tree.)

               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 Windows (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 or -t switch was
               specified, neither PERL5LIB nor PERLLIB is consulted. The
               program should instead say:

                   use lib "/my/directory";

   PERL5OPT    Command-line options (switches).  Switches in this variable
               are treated as if they were on every Perl command line.
               Only the -[CDIMUdmtwW] switches are allowed.  When running
               taint checks (either because the program was running setuid
               or setgid, or because the -T or -t switch was used), this
               variable is ignored.  If PERL5OPT begins with -T, tainting
               will be enabled and subsequent options ignored.  If
               PERL5OPT begins with -t, tainting will be enabled, a
               writable dot removed from @INC, and subsequent options

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

               It is conventional to start layer names with a colon (for
               example, ":perlio") to emphasize 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

               An unset or empty PERLIO is equivalent to the default set
               of layers for your platform; for example, ":unix:perlio" on
               Unix-like systems and ":unix:crlf" on Windows and other
               DOS-like systems.

               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.

               Layers it makes sense to include in the PERLIO environment
               variable are briefly summarized below. For more details see

               :bytes  A pseudolayer that turns the ":utf8" flag off 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

               :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

               :mmap   A layer that implements "reading" of files by using
                       mmap(2) to make an entire 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 nitroglycerine.

               :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 translation.  In particular, both
                       CRLF translation and intuiting ":utf8" from the
                       locale are disabled.

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

               :unix   Low-level layer that calls "read", "write",
                       "lseek", etc.

               :utf8   A pseudolayer that enables a flag in the layer
                       below to tell Perl that output should be in utf8
                       and that input should be regarded as already in
                       valid utf8 form. WARNING: It does not check for
                       validity and as such should be handled with extreme
                       caution for input, because security violations can
                       occur with non-shortest UTF-8 encodings, etc.
                       Generally ":encoding(utf8)" is the best option when
                       reading UTF-8 encoded data.

               :win32  On Win32 platforms this experimental layer uses
                       native "handle" IO rather than a Unix-like numeric
                       file descriptor layer. Known to be buggy in this
                       release (5.14).

               The default set of layers should give acceptable results on
               all platforms

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

               On Win32 the default in this release (5.14) is "unix crlf".
               Win32's "stdio" has a number of bugs/mis-features for Perl
               IO which are somewhat depending on the version and vendor
               of the C compiler. Using our own "crlf" layer as the buffer
               avoids those issues and makes things more uniform.  The
               "crlf" layer provides CRLF conversion as well as buffering.

               This release (5.14) uses "unix" as the bottom layer on
               Win32, and so still uses the C compiler's numeric file
               descriptor routines. There is an experimental native
               "win32" layer, which is expected to be enhanced and should
               eventually become the default under Win32.

               The PERLIO environment variable is completely ignored when
               Perl is run in taint mode.

               If set to the name of a file or device when Perl is run
               with the -Di command-line switch, the logging of certain
               operations of the PerlIO subsystem will be redirected to
               the specified file rather than going to stderr, which is
               the default. The file is opened in append mode. Typical
               uses are in Unix:

                  % env PERLIO_DEBUG=/tmp/perlio.log perl -Di script ...

               and under Win32, the approximately equivalent:

                  > set PERLIO_DEBUG=CON
                  perl -Di script ...

               This functionality is disabled for setuid scripts, for
               scripts run with -T, and for scripts run on a Perl built
               without "-DDEBUGGING" support.

   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

               The PERLLIB environment variable is completely ignored when
               Perl is run in taint mode.

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

                       BEGIN { require "" }

               The PERL5DB environment variable is only used when Perl is
               started with a bare -d switch.

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

   PERL5SHELL (specific to the Win32 port)
               On Win32 ports only, 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 space-separated.  Precede any character that
               needs to be protected, like a space or backslash, with
               another backslash.

               Note that Perl doesn't use COMSPEC for this purpose because
               COMSPEC has a high degree of variability 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
               functioning of other programs (which usually look in
               COMSPEC to find a shell fit for interactive use).

               Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint
               checked when running external commands.  It is recommended
               that you explicitly set (or delete) $ENV{PERL5SHELL} when
               running in taint mode under Windows.

   PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
               Set to 1 to allow the use of non-IFS compatible LSPs
               (Layered Service Providers).  Perl normally searches for an
               IFS-compatible LSP because this is required for its
               emulation of Windows sockets as real filehandles.  However,
               this may cause problems if you have a firewall such as
               McAfee Guardian, which requires that all applications use
               its LSP but 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
               particular case Perl still works too because McAfee
               Guardian's LSP actually plays 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 dumps out memory statistics after execution.
               If set to an integer greater than one, also dumps out
               memory statistics after compilation.

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

               Set to "1" 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 them.

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

               (Since Perl 5.8.1, new semantics in Perl 5.18.0)  Used to
               override the randomization of Perl's internal hash
               function. The value is expressed in hexadecimal, and may
               include a leading 0x. Truncated patterns are treated as
               though they are suffixed with sufficient 0's as required.

               If the option is provided, and "PERL_PERTURB_KEYS" is NOT
               set, then a value of '0' implies "PERL_PERTURB_KEYS=0" and
               any other value implies "PERL_PERTURB_KEYS=2".

               PLEASE NOTE: 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 partially or completely lost.

               See "Algorithmic Complexity Attacks" in perlsec,
               "PERL_PERTURB_KEYS", and "PERL_HASH_SEED_DEBUG" for more

               (Since Perl 5.18.0)  Set to "0" or "NO" then traversing
               keys will be repeatable from run to run for the same
               PERL_HASH_SEED.  Insertion into a hash will not change the
               order, except to provide for more space in the hash. When
               combined with setting PERL_HASH_SEED this mode is as close
               to pre 5.18 behavior as you can get.

               When set to "1" or "RANDOM" then traversing keys will be
               randomized.  Every time a hash is inserted into the key
               order will change in a random fashion. The order may not be
               repeatable in a following program run even if the
               PERL_HASH_SEED has been specified. This is the default mode
               for perl.

               When set to "2" or "DETERMINISTIC" then inserting keys into
               a hash will cause the key order to change, but in a way
               that is repeatable from program run to program run.

               NOTE: Use of this option is considered insecure, and is
               intended only for debugging non-deterministic behavior in
               Perl's hash function. Do not use it in production.

               See "Algorithmic Complexity Attacks" in perlsec and
               "PERL_HASH_SEED" and "PERL_HASH_SEED_DEBUG" for more
               information. You can get and set the key traversal mask for
               a specific hash by using the "hash_traversal_mask()"
               function from Hash::Util.

               (Since Perl 5.8.1.)  Set to "1" to display (to STDERR)
               information about the hash function, seed, and what type of
               key traversal randomization is in effect at the beginning
               of execution.  This, combined with "PERL_HASH_SEED" and
               "PERL_PERTURB_KEYS" is intended to aid in debugging
               nondeterministic behaviour caused by hash randomization.

               Note that any information about the hash function,
               especially 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()" and "key_traversal_mask()" in

               An example output might be:

                HASH_FUNCTION = ONE_AT_A_TIME_HARD HASH_SEED = 0x652e9b9349a7a032 PERTURB_KEYS = 1 (RANDOM)

               If your Perl was configured with -Accflags=-DPERL_MEM_LOG,
               setting the environment variable "PERL_MEM_LOG" enables
               logging debug messages. The value has the form
               "<number>[m][s][t]", where "number" is the file descriptor
               number you want to write to (2 is default), and the
               combination of letters specifies that you want information
               about (m)emory and/or (s)v, optionally with (t)imestamps.
               For example, "PERL_MEM_LOG=1mst" logs all information to
               stdout. You can write to other opened file descriptors in a
               variety of ways:

                 $ 3>foo3 PERL_MEM_LOG=3m perl ...

   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
               but are optional and discussed further in perlvms and in
               README.vms in the Perl source distribution.

               Available in Perls 5.8.1 and later.  If set to "unsafe",
               the pre-Perl-5.8.0 signal behaviour (which is immediate but
               unsafe) is restored.  If set to "safe", then safe (but
               deferred) signals are used.  See "Deferred Signals (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

   Perl also has environment variables that control how Perl handles data
   specific to particular natural languages; see perllocale.

   Perl and its various modules and components, including its test
   frameworks, may sometimes make use of certain other environment
   variables.  Some of these are specific to a particular platform.
   Please consult the appropriate module documentation and any
   documentation for your platform (like perlsolaris, perllinux,
   perlmacosx, perlwin32, etc) for variables peculiar to those specific

   Perl makes all environment variables available to the program being
   executed, and passes these along to any child processes it starts.
   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)};

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