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X11R7.4 - man page for perlrun (x11r4 section 1)

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

       perlrun - how to execute the Perl interpreter

       perl [ -sTtuUWX ]      [ -hv ] [ -V[:configvar] ]      [ -cw ] [ -d[t][:debug-
       ger] ] [ -D[number/list] ]      [ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal/hexadeci-
       mal] ]	   [ -Idir ] [ -m[-]module ] [ -M[-]'module...' ] [ -f ]      [ -C [num-
       ber/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 pass-
       ing 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 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

       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 con-
       taining 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 recog-
       nize the #! line, you still can get consistent switch behavior 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 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

       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, perl5.005_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 exe-
       cuted 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

       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

       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 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, click-
	   able apps can be made from any "#!" script using Wil Sanchez' DropScript utility:
	   http://www.wsanchez.net/software/ .

       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 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 follow-
       ing 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

	    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 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
		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) 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 manipu-
	    late 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 set-
	    ting.  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 execute "BEGIN", "CHECK", and "use" blocks, because these are con-
	    sidered 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

	    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 equiva-
	    lent 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
	      4194304  A  Consistency checks on internal structures
	      8388608  q  quiet - currently only suppressed 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 dis-
	    tribution 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.

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

	    Perl can be built so that it by default will try to execute $Config{sitelib}/sitecus-
	    tomize.pl 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.

	    specifies the pattern to split on if -a is also in effect.	The pattern may be sur-
	    rounded 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, fol-
	    lowing 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 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

		$ 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

	    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

	    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 com-
	    mand 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 preprocessor 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 $/ (the input record separator) when used with -n or -p.  Sec-
	    ond, it assigns "$\" (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 omit-
	    ted, sets "$\" to the current value of $/.	For instance, to trim lines to 80 col-

		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 -Mmod-
	    ule=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 exam-
	    ple if Foo inherits from Exporter.)

       -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 file.

	    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

	    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 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.

       -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 compila-
	    tion 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 mod-
	    ule 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 compli-
	    ant 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 search-
	    ing 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 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 rela-
	    tive 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 warn-
	    ings 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 neces-
	    sarily 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" exe-
	    cutable 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 pro-
	    grams 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 sup-
	    presses 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 pro-
	    mote into fatal errors specific warnings using "__WARN__" hooks, as described in per-
	    lvar and "warn" in perlfunc.  See also perldiag and perltrap.  A new, fine-grained
	    warning facility is also available if you want to manipulate entire classes of warn-
	    ings; 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.

	    tells Perl that the program is embedded in a larger chunk of unrelated ASCII 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, and not as line 100.  This
	    can be overridden by using the #line directive.  (See "Plain-Old-Comments-(Not!)" in

	    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 pro-
	    gram 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 inter-
	    vening 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
		   directories under the specified locations are automatically included if they
		   exist (this lookup being done at interpreter startup time.)

		   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 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 -[CDIMUdmtw] 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 empha-
		   sise 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 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 encod-
		   ings 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 ":per-

		   :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 plat-
			   forms normal behaviour. You will need a ":crlf" layer above it to do

		   :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 valid utf8 form. It does not check for validity and as such
			   should be handled with caution for input. 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 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".  Config-
		   ure 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-features for perl IO which are somewhat C compiler ven-
		   dor/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 com-
		   piler'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.

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

		   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.

		   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 is:

			   BEGIN { require 'perl5db.pl' }

		   The PERL5DB environment variable only used when perl is started with a bare -d

		   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
		   "command.com /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 variability among users, leading to portability concerns.	Besides,
		   perl can use a shell that may not be fit for interactive use, and setting COM-
		   SPEC to such a shell may interfere with the proper functioning of other pro-
		   grams (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 LSP's.  Perl normally searches
		   for an IFS-compatible LSP because this is required for its emulation of Win-
		   dows sockets as real filehandles.  However, 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 particular 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 dis-
		   tribution (that is, if "perl -V:d_mymalloc" is 'define').  If set, this causes
		   memory statistics to be dumped after execution.  If set to an integer greater
		   than one, also causes memory statistics to be dumped after compilation.

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

		   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 emu-
		   late 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
		   always have the same ordering between different runs of perl.

		   Most hashes return elements in the same order as Perl 5.8.0 by default.  On a
		   hash by hash basis, if pathological data is detected during a hash key inser-
		   tion, then that hash will switch to an alternative random hash seed.

		   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 pseu-
		   dorandom seed supplied by the operating system and libraries.

		   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 partially 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 nondeterministic behavior caused by hash random-

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

		   In Perls 5.8.1 and later.  If set to "unsafe" the pre-Perl-5.8.0 signals be-
		   haviour (immediate but unsafe) is restored.	If set to "safe" the safe (or
		   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" (what-
		   ever 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 partic-
       ular natural languages.	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 peo-
       ple 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 v5.8.9				    2007-11-17				       PERLRUN(1)

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