3 perlrun - how to execute the Perl interpreter
7 B<perl> S<[ B<-CsTtuUWX> ]>
8 S<[ B<-hv> ] [ B<-V>[:I<configvar>] ]>
9 S<[ B<-cw> ] [ B<-d>[:I<debugger>] ] [ B<-D>[I<number/list>] ]>
10 S<[ B<-pna> ] [ B<-F>I<pattern> ] [ B<-l>[I<octal>] ] [ B<-0>[I<octal>] ]>
11 S<[ B<-I>I<dir> ] [ B<-m>[B<->]I<module> ] [ B<-M>[B<->]I<'module...'> ]>
15 S<[ B<-i>[I<extension>] ]>
16 S<[ B<-e> I<'command'> ] [ B<--> ] [ I<programfile> ] [ I<argument> ]...>
20 The normal way to run a Perl program is by making it directly
21 executable, or else by passing the name of the source file as an
22 argument on the command line. (An interactive Perl environment
23 is also possible--see L<perldebug> for details on how to do that.)
24 Upon startup, Perl looks for your program in one of the following
31 Specified line by line via B<-e> switches on the command line.
35 Contained in the file specified by the first filename on the command line.
36 (Note that systems supporting the #! notation invoke interpreters this
37 way. See L<Location of Perl>.)
41 Passed in implicitly via standard input. This works only if there are
42 no filename arguments--to pass arguments to a STDIN-read program you
43 must explicitly specify a "-" for the program name.
47 With methods 2 and 3, Perl starts parsing the input file from the
48 beginning, unless you've specified a B<-x> switch, in which case it
49 scans for the first line starting with #! and containing the word
50 "perl", and starts there instead. This is useful for running a program
51 embedded in a larger message. (In this case you would indicate the end
52 of the program using the C<__END__> token.)
54 The #! line is always examined for switches as the line is being
55 parsed. Thus, if you're on a machine that allows only one argument
56 with the #! line, or worse, doesn't even recognize the #! line, you
57 still can get consistent switch behavior regardless of how Perl was
58 invoked, even if B<-x> was used to find the beginning of the program.
60 Because historically some operating systems silently chopped off
61 kernel interpretation of the #! line after 32 characters, some
62 switches may be passed in on the command line, and some may not;
63 you could even get a "-" without its letter, if you're not careful.
64 You probably want to make sure that all your switches fall either
65 before or after that 32-character boundary. Most switches don't
66 actually care if they're processed redundantly, but getting a "-"
67 instead of a complete switch could cause Perl to try to execute
68 standard input instead of your program. And a partial B<-I> switch
69 could also cause odd results.
71 Some switches do care if they are processed twice, for instance
72 combinations of B<-l> and B<-0>. Either put all the switches after
73 the 32-character boundary (if applicable), or replace the use of
74 B<-0>I<digits> by C<BEGIN{ $/ = "\0digits"; }>.
76 Parsing of the #! switches starts wherever "perl" is mentioned in the line.
77 The sequences "-*" and "- " are specifically ignored so that you could,
78 if you were so inclined, say
80 #!/bin/sh -- # -*- perl -*- -p
81 eval 'exec perl -wS $0 ${1+"$@"}'
82 if $running_under_some_shell;
84 to let Perl see the B<-p> switch.
86 A similar trick involves the B<env> program, if you have it.
90 The examples above use a relative path to the perl interpreter,
91 getting whatever version is first in the user's path. If you want
92 a specific version of Perl, say, perl5.005_57, you should place
93 that directly in the #! line's path.
95 If the #! line does not contain the word "perl", the program named after
96 the #! is executed instead of the Perl interpreter. This is slightly
97 bizarre, but it helps people on machines that don't do #!, because they
98 can tell a program that their SHELL is F</usr/bin/perl>, and Perl will then
99 dispatch the program to the correct interpreter for them.
101 After locating your program, Perl compiles the entire program to an
102 internal form. If there are any compilation errors, execution of the
103 program is not attempted. (This is unlike the typical shell script,
104 which might run part-way through before finding a syntax error.)
106 If the program is syntactically correct, it is executed. If the program
107 runs off the end without hitting an exit() or die() operator, an implicit
108 C<exit(0)> is provided to indicate successful completion.
110 =head2 #! and quoting on non-Unix systems
112 Unix's #! technique can be simulated on other systems:
120 extproc perl -S -your_switches
122 as the first line in C<*.cmd> file (B<-S> due to a bug in cmd.exe's
127 Create a batch file to run your program, and codify it in
128 C<ALTERNATIVE_SHEBANG> (see the F<dosish.h> file in the source
129 distribution for more information).
133 The Win95/NT installation, when using the ActiveState installer for Perl,
134 will modify the Registry to associate the F<.pl> extension with the perl
135 interpreter. If you install Perl by other means (including building from
136 the sources), you may have to modify the Registry yourself. Note that
137 this means you can no longer tell the difference between an executable
138 Perl program and a Perl library file.
142 A Macintosh perl program will have the appropriate Creator and
143 Type, so that double-clicking them will invoke the perl application.
149 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
150 $ exit++ + ++$status != 0 and $exit = $status = undef;
152 at the top of your program, where B<-mysw> are any command line switches you
153 want to pass to Perl. You can now invoke the program directly, by saying
154 C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
155 via F<DCL$PATH> by just using the name of the program).
157 This incantation is a bit much to remember, but Perl will display it for
158 you if you say C<perl "-V:startperl">.
162 Command-interpreters on non-Unix systems have rather different ideas
163 on quoting than Unix shells. You'll need to learn the special
164 characters in your command-interpreter (C<*>, C<\> and C<"> are
165 common) and how to protect whitespace and these characters to run
166 one-liners (see B<-e> below).
168 On some systems, you may have to change single-quotes to double ones,
169 which you must I<not> do on Unix or Plan9 systems. You might also
170 have to change a single % to a %%.
175 perl -e 'print "Hello world\n"'
178 perl -e "print \"Hello world\n\""
181 print "Hello world\n"
182 (then Run "Myscript" or Shift-Command-R)
185 perl -e "print ""Hello world\n"""
187 The problem is that none of this is reliable: it depends on the
188 command and it is entirely possible neither works. If B<4DOS> were
189 the command shell, this would probably work better:
191 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
193 B<CMD.EXE> in Windows NT slipped a lot of standard Unix functionality in
194 when nobody was looking, but just try to find documentation for its
197 Under the Macintosh, it depends which environment you are using. The MacPerl
198 shell, or MPW, is much like Unix shells in its support for several
199 quoting variants, except that it makes free use of the Macintosh's non-ASCII
200 characters as control characters.
202 There is no general solution to all of this. It's just a mess.
204 =head2 Location of Perl
206 It may seem obvious to say, but Perl is useful only when users can
207 easily find it. When possible, it's good for both F</usr/bin/perl>
208 and F</usr/local/bin/perl> to be symlinks to the actual binary. If
209 that can't be done, system administrators are strongly encouraged
210 to put (symlinks to) perl and its accompanying utilities into a
211 directory typically found along a user's PATH, or in some other
212 obvious and convenient place.
214 In this documentation, C<#!/usr/bin/perl> on the first line of the program
215 will stand in for whatever method works on your system. You are
216 advised to use a specific path if you care about a specific version.
218 #!/usr/local/bin/perl5.00554
220 or if you just want to be running at least version, place a statement
221 like this at the top of your program:
225 =head2 Command Switches
227 As with all standard commands, a single-character switch may be
228 clustered with the following switch, if any.
230 #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
236 =item B<-0>[I<digits>]
238 specifies the input record separator (C<$/>) as an octal number. If there are
239 no digits, the null character is the separator. Other switches may
240 precede or follow the digits. For example, if you have a version of
241 B<find> which can print filenames terminated by the null character, you
244 find . -name '*.orig' -print0 | perl -n0e unlink
246 The special value 00 will cause Perl to slurp files in paragraph mode.
247 The value 0777 will cause Perl to slurp files whole because there is no
248 legal character with that value.
252 turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
253 split command to the @F array is done as the first thing inside the
254 implicit while loop produced by the B<-n> or B<-p>.
256 perl -ane 'print pop(@F), "\n";'
265 An alternate delimiter may be specified using B<-F>.
269 enables Perl to use the native wide character APIs on the target system.
270 The magic variable C<${^WIDE_SYSTEM_CALLS}> reflects the state of
271 this switch. See L<perlvar/"${^WIDE_SYSTEM_CALLS}">.
273 This feature is currently only implemented on the Win32 platform.
277 causes Perl to check the syntax of the program and then exit without
278 executing it. Actually, it I<will> execute C<BEGIN>, C<CHECK>, and
279 C<use> blocks, because these are considered as occurring outside the
280 execution of your program. C<INIT> and C<END> blocks, however, will
285 runs the program under the Perl debugger. See L<perldebug>.
287 =item B<-d:>I<foo[=bar,baz]>
289 runs the program under the control of a debugging, profiling, or
290 tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
291 the program using the Devel::DProf profiler. As with the B<-M>
292 flag, options may be passed to the Devel::foo package where they
293 will be received and interpreted by the Devel::foo::import routine.
294 The comma-separated list of options must follow a C<=> character.
297 =item B<-D>I<letters>
301 sets debugging flags. To watch how it executes your program, use
302 B<-Dtls>. (This works only if debugging is compiled into your
303 Perl.) Another nice value is B<-Dx>, which lists your compiled
304 syntax tree. And B<-Dr> displays compiled regular expressions. As an
305 alternative, specify a number instead of list of letters (e.g., B<-D14> is
306 equivalent to B<-Dtls>):
308 1 p Tokenizing and parsing
310 4 l Context (loop) stack processing
312 16 o Method and overloading resolution
313 32 c String/numeric conversions
314 64 P Print preprocessor command for -P, source file input state
315 128 m Memory allocation
316 256 f Format processing
317 512 r Regular expression parsing and execution
318 1024 x Syntax tree dump
319 2048 u Tainting checks
320 4096 L Memory leaks (needs -DLEAKTEST when compiling Perl)
321 8192 H Hash dump -- usurps values()
322 16384 X Scratchpad allocation
324 65536 S Thread synchronization
326 262144 R Include reference counts of dumped variables (eg when using -Ds)
328 All these flags require B<-DDEBUGGING> when you compile the Perl
329 executable. See the F<INSTALL> file in the Perl source distribution
330 for how to do this. This flag is automatically set if you include B<-g>
331 option when C<Configure> asks you about optimizer/debugger flags.
333 If you're just trying to get a print out of each line of Perl code
334 as it executes, the way that C<sh -x> provides for shell scripts,
335 you can't use Perl's B<-D> switch. Instead do this
337 # Bourne shell syntax
338 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
341 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
343 See L<perldebug> for details and variations.
345 =item B<-e> I<commandline>
347 may be used to enter one line of program. If B<-e> is given, Perl
348 will not look for a filename in the argument list. Multiple B<-e>
349 commands may be given to build up a multi-line script. Make sure
350 to use semicolons where you would in a normal program.
352 =item B<-F>I<pattern>
354 specifies the pattern to split on if B<-a> is also in effect. The
355 pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
356 put in single quotes.
360 prints a summary of the options.
362 =item B<-i>[I<extension>]
364 specifies that files processed by the C<E<lt>E<gt>> construct are to be
365 edited in-place. It does this by renaming the input file, opening the
366 output file by the original name, and selecting that output file as the
367 default for print() statements. The extension, if supplied, is used to
368 modify the name of the old file to make a backup copy, following these
371 If no extension is supplied, no backup is made and the current file is
374 If the extension doesn't contain a C<*>, then it is appended to the
375 end of the current filename as a suffix. If the extension does
376 contain one or more C<*> characters, then each C<*> is replaced
377 with the current filename. In Perl terms, you could think of this
380 ($backup = $extension) =~ s/\*/$file_name/g;
382 This allows you to add a prefix to the backup file, instead of (or in
383 addition to) a suffix:
385 $ perl -pi 'orig_*' -e 's/bar/baz/' fileA # backup to 'orig_fileA'
387 Or even to place backup copies of the original files into another
388 directory (provided the directory already exists):
390 $ perl -pi 'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
392 These sets of one-liners are equivalent:
394 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
395 $ perl -pi '*' -e 's/bar/baz/' fileA # overwrite current file
397 $ perl -pi '.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
398 $ perl -pi '*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
400 From the shell, saying
402 $ perl -p -i.orig -e "s/foo/bar/; ... "
404 is the same as using the program:
406 #!/usr/bin/perl -pi.orig
409 which is equivalent to
412 $extension = '.orig';
414 if ($ARGV ne $oldargv) {
415 if ($extension !~ /\*/) {
416 $backup = $ARGV . $extension;
419 ($backup = $extension) =~ s/\*/$ARGV/g;
421 rename($ARGV, $backup);
422 open(ARGVOUT, ">$ARGV");
429 print; # this prints to original filename
433 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
434 know when the filename has changed. It does, however, use ARGVOUT for
435 the selected filehandle. Note that STDOUT is restored as the default
436 output filehandle after the loop.
438 As shown above, Perl creates the backup file whether or not any output
439 is actually changed. So this is just a fancy way to copy files:
441 $ perl -p -i '/some/file/path/*' -e 1 file1 file2 file3...
443 $ perl -p -i '.orig' -e 1 file1 file2 file3...
445 You can use C<eof> without parentheses to locate the end of each input
446 file, in case you want to append to each file, or reset line numbering
447 (see example in L<perlfunc/eof>).
449 If, for a given file, Perl is unable to create the backup file as
450 specified in the extension then it will skip that file and continue on
451 with the next one (if it exists).
453 For a discussion of issues surrounding file permissions and B<-i>,
454 see L<perlfaq5/Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl?>.
456 You cannot use B<-i> to create directories or to strip extensions from
459 Perl does not expand C<~> in filenames, which is good, since some
460 folks use it for their backup files:
462 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
464 Finally, the B<-i> switch does not impede execution when no
465 files are given on the command line. In this case, no backup is made
466 (the original file cannot, of course, be determined) and processing
467 proceeds from STDIN to STDOUT as might be expected.
469 =item B<-I>I<directory>
471 Directories specified by B<-I> are prepended to the search path for
472 modules (C<@INC>), and also tells the C preprocessor where to search for
473 include files. The C preprocessor is invoked with B<-P>; by default it
474 searches /usr/include and /usr/lib/perl.
476 =item B<-l>[I<octnum>]
478 enables automatic line-ending processing. It has two separate
479 effects. First, it automatically chomps C<$/> (the input record
480 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
481 (the output record separator) to have the value of I<octnum> so
482 that any print statements will have that separator added back on.
483 If I<octnum> is omitted, sets C<$\> to the current value of
484 C<$/>. For instance, to trim lines to 80 columns:
486 perl -lpe 'substr($_, 80) = ""'
488 Note that the assignment C<$\ = $/> is done when the switch is processed,
489 so the input record separator can be different than the output record
490 separator if the B<-l> switch is followed by a B<-0> switch:
492 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
494 This sets C<$\> to newline and then sets C<$/> to the null character.
496 =item B<-m>[B<->]I<module>
498 =item B<-M>[B<->]I<module>
500 =item B<-M>[B<->]I<'module ...'>
502 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
504 B<-m>I<module> executes C<use> I<module> C<();> before executing your
507 B<-M>I<module> executes C<use> I<module> C<;> before executing your
508 program. You can use quotes to add extra code after the module name,
509 e.g., C<'-Mmodule qw(foo bar)'>.
511 If the first character after the B<-M> or B<-m> is a dash (C<->)
512 then the 'use' is replaced with 'no'.
514 A little builtin syntactic sugar means you can also say
515 B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
516 C<'-Mmodule qw(foo bar)'>. This avoids the need to use quotes when
517 importing symbols. The actual code generated by B<-Mmodule=foo,bar> is
518 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
519 removes the distinction between B<-m> and B<-M>.
523 causes Perl to assume the following loop around your program, which
524 makes it iterate over filename arguments somewhat like B<sed -n> or
529 ... # your program goes here
532 Note that the lines are not printed by default. See B<-p> to have
533 lines printed. If a file named by an argument cannot be opened for
534 some reason, Perl warns you about it and moves on to the next file.
536 Here is an efficient way to delete all files older than a week:
538 find . -mtime +7 -print | perl -nle unlink
540 This is faster than using the B<-exec> switch of B<find> because you don't
541 have to start a process on every filename found. It does suffer from
542 the bug of mishandling newlines in pathnames, which you can fix if
545 C<BEGIN> and C<END> blocks may be used to capture control before or after
546 the implicit program loop, just as in B<awk>.
550 causes Perl to assume the following loop around your program, which
551 makes it iterate over filename arguments somewhat like B<sed>:
556 ... # your program goes here
558 print or die "-p destination: $!\n";
561 If a file named by an argument cannot be opened for some reason, Perl
562 warns you about it, and moves on to the next file. Note that the
563 lines are printed automatically. An error occurring during printing is
564 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
565 overrides a B<-n> switch.
567 C<BEGIN> and C<END> blocks may be used to capture control before or after
568 the implicit loop, just as in B<awk>.
572 B<NOTE: Use of -P is strongly discouraged because of its inherent
573 problems, including poor portability.>
575 This option causes your program to be run through the C preprocessor before
576 compilation by Perl. Because both comments and B<cpp> directives begin
577 with the # character, you should avoid starting comments with any words
578 recognized by the C preprocessor such as C<"if">, C<"else">, or C<"define">.
580 If you're considering using C<-P>, you might also want to look at the
581 Filter::cpp module from CPAN.
583 The problems of -P include, but are not limited to:
589 The C<#!> line is stripped, so any switches there don't apply.
593 A C<-P> on a C<#!> line doesn't work.
597 B<All> lines that begin with (whitespace and) a C<#> but
598 do not look like cpp commands, are stripped, including anything
599 inside Perl strings, regular expressions, and here-docs .
603 In some platforms the C preprocessor knows too much: it knows about
604 the C++ -style until-end-of-line comments starting with C<"//">.
605 This will cause problems with common Perl constructs like
609 because after -P this will became illegal code
613 The workaround is to use some other quoting separator than C<"/">,
614 like for example C<"!">:
622 It requires not only a working C preprocessor but also a working
623 F<sed>. If not on UNIX, you are probably out of luck on this.
627 Script line numbers are not preserved.
631 The C<-x> does not work with C<-P>.
637 enables rudimentary switch parsing for switches on the command
638 line after the program name but before any filename arguments (or before
639 an argument of B<-->). This means you can have switches with two leading
640 dashes (B<--help>). Any switch found there is removed from @ARGV and sets the
641 corresponding variable in the Perl program. The following program
642 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
643 if it is invoked with B<-xyz=abc>.
646 if ($xyz) { print "$xyz\n" }
648 Do note that B<--help> creates the variable ${-help}, which is not compliant
653 makes Perl use the PATH environment variable to search for the
654 program (unless the name of the program contains directory separators).
656 On some platforms, this also makes Perl append suffixes to the
657 filename while searching for it. For example, on Win32 platforms,
658 the ".bat" and ".cmd" suffixes are appended if a lookup for the
659 original name fails, and if the name does not already end in one
660 of those suffixes. If your Perl was compiled with DEBUGGING turned
661 on, using the -Dp switch to Perl shows how the search progresses.
663 Typically this is used to emulate #! startup on platforms that
664 don't support #!. This example works on many platforms that
665 have a shell compatible with Bourne shell:
668 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
669 if $running_under_some_shell;
671 The system ignores the first line and feeds the program to F</bin/sh>,
672 which proceeds to try to execute the Perl program as a shell script.
673 The shell executes the second line as a normal shell command, and thus
674 starts up the Perl interpreter. On some systems $0 doesn't always
675 contain the full pathname, so the B<-S> tells Perl to search for the
676 program if necessary. After Perl locates the program, it parses the
677 lines and ignores them because the variable $running_under_some_shell
678 is never true. If the program will be interpreted by csh, you will need
679 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
680 embedded spaces (and such) in the argument list. To start up sh rather
681 than csh, some systems may have to replace the #! line with a line
682 containing just a colon, which will be politely ignored by Perl. Other
683 systems can't control that, and need a totally devious construct that
684 will work under any of B<csh>, B<sh>, or Perl, such as the following:
686 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
687 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
688 if $running_under_some_shell;
690 If the filename supplied contains directory separators (i.e., is an
691 absolute or relative pathname), and if that file is not found,
692 platforms that append file extensions will do so and try to look
693 for the file with those extensions added, one by one.
695 On DOS-like platforms, if the program does not contain directory
696 separators, it will first be searched for in the current directory
697 before being searched for on the PATH. On Unix platforms, the
698 program will be searched for strictly on the PATH.
702 Like B<-T>, but taint checks will issue warnings rather than fatal
703 errors. Since these are warnings, the B<-w> switch (or C<use
704 warnings>) must be used along with this option. B<NOTE: this is
705 not a substitute for -T.> This is meant only to be used as a temporary
706 aid while securing legacy code: for real production code and for new
707 secure code written from scratch always use the real B<-T>.
711 forces "taint" checks to be turned on so you can test them. Ordinarily
712 these checks are done only when running setuid or setgid. It's a
713 good idea to turn them on explicitly for programs that run on behalf
714 of someone else whom you might not necessarily trust, such as CGI
715 programs or any internet servers you might write in Perl. See
716 L<perlsec> for details. For security reasons, this option must be
717 seen by Perl quite early; usually this means it must appear early
718 on the command line or in the #! line for systems which support
723 This obsolete switch causes Perl to dump core after compiling your
724 program. You can then in theory take this core dump and turn it
725 into an executable file by using the B<undump> program (not supplied).
726 This speeds startup at the expense of some disk space (which you
727 can minimize by stripping the executable). (Still, a "hello world"
728 executable comes out to about 200K on my machine.) If you want to
729 execute a portion of your program before dumping, use the dump()
730 operator instead. Note: availability of B<undump> is platform
731 specific and may not be available for a specific port of Perl.
733 This switch has been superseded in favor of the new Perl code
734 generator backends to the compiler. See L<B> and L<B::Bytecode>
739 allows Perl to do unsafe operations. Currently the only "unsafe"
740 operations are the unlinking of directories while running as superuser,
741 and running setuid programs with fatal taint checks turned into
742 warnings. Note that the B<-w> switch (or the C<$^W> variable) must
743 be used along with this option to actually I<generate> the
744 taint-check warnings.
748 prints the version and patchlevel of your perl executable.
752 prints summary of the major perl configuration values and the current
757 Prints to STDOUT the value of the named configuration variable.
762 will provide strong clues about what your MANPATH variable should
763 be set to in order to access the Perl documentation.
767 prints warnings about dubious constructs, such as variable names
768 that are mentioned only once and scalar variables that are used
769 before being set, redefined subroutines, references to undefined
770 filehandles or filehandles opened read-only that you are attempting
771 to write on, values used as a number that doesn't look like numbers,
772 using an array as though it were a scalar, if your subroutines
773 recurse more than 100 deep, and innumerable other things.
775 This switch really just enables the internal C<^$W> variable. You
776 can disable or promote into fatal errors specific warnings using
777 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
778 See also L<perldiag> and L<perltrap>. A new, fine-grained warning
779 facility is also available if you want to manipulate entire classes
780 of warnings; see L<warnings> or L<perllexwarn>.
784 Enables all warnings regardless of C<no warnings> or C<$^W>.
789 Disables all warnings regardless of C<use warnings> or C<$^W>.
792 =item B<-x> I<directory>
794 tells Perl that the program is embedded in a larger chunk of unrelated
795 ASCII text, such as in a mail message. Leading garbage will be
796 discarded until the first line that starts with #! and contains the
797 string "perl". Any meaningful switches on that line will be applied.
798 If a directory name is specified, Perl will switch to that directory
799 before running the program. The B<-x> switch controls only the
800 disposal of leading garbage. The program must be terminated with
801 C<__END__> if there is trailing garbage to be ignored (the program
802 can process any or all of the trailing garbage via the DATA filehandle
813 Used if chdir has no argument.
817 Used if chdir has no argument and HOME is not set.
821 Used in executing subprocesses, and in finding the program if B<-S> is
826 A colon-separated list of directories in which to look for Perl library
827 files before looking in the standard library and the current
828 directory. Any architecture-specific directories under the specified
829 locations are automatically included if they exist. If PERL5LIB is not
830 defined, PERLLIB is used.
832 When running taint checks (either because the program was running setuid
833 or setgid, or the B<-T> switch was used), neither variable is used.
834 The program should instead say:
836 use lib "/my/directory";
840 Command-line options (switches). Switches in this variable are taken
841 as if they were on every Perl command line. Only the B<-[DIMUdmtw]>
842 switches are allowed. When running taint checks (because the program
843 was running setuid or setgid, or the B<-T> switch was used), this
844 variable is ignored. If PERL5OPT begins with B<-T>, tainting will be
845 enabled, and any subsequent options ignored.
849 A colon-separated list of directories in which to look for Perl library
850 files before looking in the standard library and the current directory.
851 If PERL5LIB is defined, PERLLIB is not used.
855 The command used to load the debugger code. The default is:
857 BEGIN { require 'perl5db.pl' }
859 =item PERL5SHELL (specific to the Win32 port)
861 May be set to an alternative shell that perl must use internally for
862 executing "backtick" commands or system(). Default is C<cmd.exe /x/c>
863 on WindowsNT and C<command.com /c> on Windows95. The value is considered
864 to be space-separated. Precede any character that needs to be protected
865 (like a space or backslash) with a backslash.
867 Note that Perl doesn't use COMSPEC for this purpose because
868 COMSPEC has a high degree of variability among users, leading to
869 portability concerns. Besides, perl can use a shell that may not be
870 fit for interactive use, and setting COMSPEC to such a shell may
871 interfere with the proper functioning of other programs (which usually
872 look in COMSPEC to find a shell fit for interactive use).
874 =item PERL_DEBUG_MSTATS
876 Relevant only if perl is compiled with the malloc included with the perl
877 distribution (that is, if C<perl -V:d_mymalloc> is 'define').
878 If set, this causes memory statistics to be dumped after execution. If set
879 to an integer greater than one, also causes memory statistics to be dumped
882 =item PERL_DESTRUCT_LEVEL
884 Relevant only if your perl executable was built with B<-DDEBUGGING>,
885 this controls the behavior of global destruction of objects and other
886 references. See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
890 If using the C<encoding> pragma without an explicit encoding name, the
891 PERL_ENCODING environment variable is consulted for an encoding name.
893 =item PERL_ROOT (specific to the VMS port)
895 A translation concealed rooted logical name that contains perl and the
896 logical device for the @INC path on VMS only. Other logical names that
897 affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
898 SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
899 L<perlvms> and in F<README.vms> in the Perl source distribution.
901 =item SYS$LOGIN (specific to the VMS port)
903 Used if chdir has no argument and HOME and LOGDIR are not set.
907 Perl also has environment variables that control how Perl handles data
908 specific to particular natural languages. See L<perllocale>.
910 Apart from these, Perl uses no other environment variables, except
911 to make them available to the program being executed, and to child
912 processes. However, programs running setuid would do well to execute
913 the following lines before doing anything else, just to keep people
916 $ENV{PATH} = '/bin:/usr/bin'; # or whatever you need
917 $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
918 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};