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+=head1 NAME
+
+perlrun - how to execute the Perl interpreter
+
+=head1 SYNOPSIS
+
+B<perl> S<[ B<-sTtuUWX> ]>
+ S<[ B<-hv> ] [ B<-V>[:I<configvar>] ]>
+ S<[ B<-cw> ] [ B<-d>[B<t>][:I<debugger>] ] [ B<-D>[I<number/list>] ]>
+ S<[ B<-pna> ] [ B<-F>I<pattern> ] [ B<-l>[I<octal>] ] [ B<-0>[I<octal/hexadecimal>] ]>
+ S<[ B<-I>I<dir> ] [ B<-m>[B<->]I<module> ] [ B<-M>[B<->]I<'module...'> ] [ B<-f> ]>
+ S<[ B<-C [I<number/list>] >]>
+ S<[ B<-P> ]>
+ S<[ B<-S> ]>
+ S<[ B<-x>[I<dir>] ]>
+ S<[ B<-i>[I<extension>] ]>
+ S<[ [B<-e>|B<-E>] I<'command'> ] [ B<--> ] [ I<programfile> ] [ I<argument> ]...>
+
+=head1 DESCRIPTION
+
+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 L<perldebug> for details on how to do that.)
+Upon startup, Perl looks for your program in one of the following
+places:
+
+=over 4
+
+=item 1.
+
+Specified line by line via B<-e> or B<-E> switches on the command line.
+
+=item 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 L<Location of Perl>.)
+
+=item 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.
+
+=back
+
+With methods 2 and 3, Perl starts parsing the input file from the
+beginning, unless you've specified a B<-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 C<__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 behavior regardless of how Perl was
+invoked, even if B<-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 B<-I> switch
+could also cause odd results.
+
+Some switches do care if they are processed twice, for instance
+combinations of B<-l> and B<-0>. Either put all the switches after
+the 32-character boundary (if applicable), or replace the use of
+B<-0>I<digits> by C<BEGIN{ $/ = "\0digits"; }>.
+
+Parsing of the #! switches starts wherever "perl" is mentioned in the line.
+The sequences "-*" and "- " are specifically ignored so that you could,
+if you were so inclined, say
+
+ #!/bin/sh
+ #! -*-perl-*-
+ eval 'exec perl -x -wS $0 ${1+"$@"}'
+ if 0;
+
+to let Perl see the B<-p> switch.
+
+A similar trick involves the B<env> program, if you have it.
+
+ #!/usr/bin/env perl
+
+The examples above use a relative path to the perl interpreter,
+getting whatever version is first in the user's path. If you want
+a specific version of Perl, say, perl5.005_57, you should place
+that directly in the #! line's path.
+
+If the #! line does not contain the word "perl", the program named after
+the #! is executed instead of the Perl interpreter. This is slightly
+bizarre, but it helps people on machines that don't do #!, because they
+can tell a program that their SHELL is F</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
+C<exit(0)> is provided to indicate successful completion.
+
+=head2 #! and quoting on non-Unix systems
+X<hashbang> X<#!>
+
+Unix's #! technique can be simulated on other systems:
+
+=over 4
+
+=item OS/2
+
+Put
+
+ extproc perl -S -your_switches
+
+as the first line in C<*.cmd> file (B<-S> due to a bug in cmd.exe's
+`extproc' handling).
+
+=item MS-DOS
+
+Create a batch file to run your program, and codify it in
+C<ALTERNATE_SHEBANG> (see the F<dosish.h> file in the source
+distribution for more information).
+
+=item Win95/NT
+
+The Win95/NT installation, when using the ActiveState installer for Perl,
+will modify the Registry to associate the F<.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.
+
+=item Macintosh
+
+Under "Classic" MacOS, a perl program will have the appropriate Creator and
+Type, so that double-clicking them will invoke the MacPerl application.
+Under Mac OS X, clickable apps can be made from any C<#!> script using Wil
+Sanchez' DropScript utility: http://www.wsanchez.net/software/ .
+
+=item VMS
+
+Put
+
+ $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
+ $ exit++ + ++$status != 0 and $exit = $status = undef;
+
+at the top of your program, where B<-mysw> are any command line switches you
+want to pass to Perl. You can now invoke the program directly, by saying
+C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
+via F<DCL$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 C<perl "-V:startperl">.
+
+=back
+
+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 (C<*>, C<\> and C<"> are
+common) and how to protect whitespace and these characters to run
+one-liners (see B<-e> below).
+
+On some systems, you may have to change single-quotes to double ones,
+which you must I<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 B<4DOS> were
+the command shell, this would probably work better:
+
+ perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
+
+B<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.
+
+=head2 Location of Perl
+X<perl, location of interpreter>
+
+It may seem obvious to say, but Perl is useful only when users can
+easily find it. When possible, it's good for both F</usr/bin/perl>
+and F</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, C<#!/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.
+
+ #!/usr/local/bin/perl5.00554
+
+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;
+
+=head2 Command Switches
+X<perl, command switches> X<command switches>
+
+As with all standard commands, a single-character switch may be
+clustered with the following switch, if any.
+
+ #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
+
+Switches include:
+
+=over 5
+
+=item B<-0>[I<octal/hexadecimal>]
+X<-0> X<$/>
+
+specifies the input record separator (C<$/>) 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 B<find> which can print filenames
+terminated by the null character, you can say this:
+
+ find . -name '*.orig' -print0 | perl -n0e unlink
+
+The special value 00 will cause Perl to slurp files in paragraph mode.
+The value 0777 will cause Perl to slurp files whole because there is no
+legal byte with that value.
+
+If you want to specify any Unicode character, use the hexadecimal
+format: C<-0xHHH...>, where the C<H> are valid hexadecimal digits.
+(This means that you cannot use the C<-x> with a directory name that
+consists of hexadecimal digits.)
+
+=item B<-a>
+X<-a> X<autosplit>
+
+turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
+split command to the @F array is done as the first thing inside the
+implicit while loop produced by the B<-n> or B<-p>.
+
+ perl -ane 'print pop(@F), "\n";'
+
+is equivalent to
+
+ while (<>) {
+ @F = split(' ');
+ print pop(@F), "\n";
+ }
+
+An alternate delimiter may be specified using B<-F>.
+
+=item B<-C [I<number/list>]>
+X<-C>
+
+The C<-C> flag controls some of the Perl Unicode features.
+
+As of 5.8.1, the C<-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 ${^UTF8CACHE} to -1, to run the UTF-8 caching code in
+ debugging mode.
+
+=for documenting_the_underdocumented
+perl.h gives W/128 as PERL_UNICODE_WIDESYSCALLS "/* for Sarathy */"
+
+=for todo
+perltodo mentions Unicode in %ENV and filenames. I guess that these will be
+options e and f (or F).
+
+For example, C<-COE> and C<-C6> will both turn on UTF-8-ness on both
+STDOUT and STDERR. Repeating letters is just redundant, not cumulative
+nor toggling.
+
+The C<io> options mean that any subsequent open() (or similar I/O
+operations) will have the C<:utf8> PerlIO layer implicitly applied
+to them, in other words, UTF-8 is expected from any input stream,
+and UTF-8 is produced to any output stream. This is just the default,
+with explicit layers in open() and with binmode() one can manipulate
+streams as usual.
+
+C<-C> on its own (not followed by any number or option list), or the
+empty string C<""> for the C<PERL_UNICODE> environment variable, has the
+same effect as C<-CSDL>. In other words, the standard I/O handles and
+the default C<open()> layer are UTF-8-fied B<but> only if the locale
+environment variables indicate a UTF-8 locale. This behaviour follows
+the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
+
+You can use C<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
+disable all the above Unicode features.
+
+The read-only magic variable C<${^UNICODE}> reflects the numeric value
+of this setting. This is variable is set during Perl startup and is
+thereafter read-only. If you want runtime effects, use the three-arg
+open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
+and the C<open> pragma (see L<open>).
+
+(In Perls earlier than 5.8.1 the C<-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".)
+
+B<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.
+
+=item B<-c>
+X<-c>
+
+causes Perl to check the syntax of the program and then exit without
+executing it. Actually, it I<will> execute C<BEGIN>, C<UNITCHECK>,
+C<CHECK>, and C<use> blocks, because these are considered as occurring
+outside the execution of your program. C<INIT> and C<END> blocks,
+however, will be skipped.
+
+=item B<-d>
+X<-d> X<-dt>
+
+=item B<-dt>
+
+runs the program under the Perl debugger. See L<perldebug>.
+If B<t> is specified, it indicates to the debugger that threads
+will be used in the code being debugged.
+
+=item B<-d:>I<foo[=bar,baz]>
+X<-d> X<-dt>
+
+=item B<-dt:>I<foo[=bar,baz]>
+
+runs the program under the control of a debugging, profiling, or
+tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
+the program using the Devel::DProf profiler. As with the B<-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 C<=> character.
+If B<t> is specified, it indicates to the debugger that threads
+will be used in the code being debugged.
+See L<perldebug>.
+
+=item B<-D>I<letters>
+X<-D> X<DEBUGGING> X<-DDEBUGGING>
+
+=item B<-D>I<number>
+
+sets debugging flags. To watch how it executes your program, use
+B<-Dtls>. (This works only if debugging is compiled into your
+Perl.) Another nice value is B<-Dx>, which lists your compiled
+syntax tree. And B<-Dr> displays compiled regular expressions;
+the format of the output is explained in L<perldebguts>.
+
+As an alternative, specify a number instead of list of letters (e.g.,
+B<-D14> is equivalent to B<-Dtls>):
+
+ 1 p Tokenizing and parsing (with v, displays parse stack)
+ 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 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 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
+ 2097152 C Copy On Write
+ 4194304 A Consistency checks on internal structures
+ 8388608 q quiet - currently only suppresses the "EXECUTING" message
+
+All these flags require B<-DDEBUGGING> when you compile the Perl
+executable (but see L<Devel::Peek>, L<re> which may change this).
+See the F<INSTALL> file in the Perl source distribution
+for how to do this. This flag is automatically set if you include B<-g>
+option when C<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 C<sh -x> provides for shell scripts,
+you can't use Perl's B<-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 L<perldebug> for details and variations.
+
+=item B<-e> I<commandline>
+X<-e>
+
+may be used to enter one line of program. If B<-e> is given, Perl
+will not look for a filename in the argument list. Multiple B<-e>
+commands may be given to build up a multi-line script. Make sure
+to use semicolons where you would in a normal program.
+
+=item B<-E> I<commandline>
+X<-E>
+
+behaves just like B<-e>, except that it implicitly enables all
+optional features (in the main compilation unit). See L<feature>.
+
+=item B<-f>
+X<-f>
+
+Disable executing F<$Config{sitelib}/sitecustomize.pl> at startup.
+
+Perl can be built so that it by default will try to execute
+F<$Config{sitelib}/sitecustomize.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.
+
+=item B<-F>I<pattern>
+X<-F>
+
+specifies the pattern to split on if B<-a> is also in effect. The
+pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
+put in single quotes. You can't use literal whitespace in the pattern.
+
+=item B<-h>
+X<-h>
+
+prints a summary of the options.
+
+=item B<-i>[I<extension>]
+X<-i> X<in-place>
+
+specifies that files processed by the C<E<lt>E<gt>> construct are to be
+edited in-place. It does this by renaming the input file, opening the
+output file by the original name, and selecting that output file as the
+default for print() statements. The extension, if supplied, is used to
+modify the name of the old file to make a backup copy, following these
+rules:
+
+If no extension is supplied, no backup is made and the current file is
+overwritten.
+
+If the extension doesn't contain a C<*>, then it is appended to the
+end of the current filename as a suffix. If the extension does
+contain one or more C<*> characters, then each C<*> 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
+ s/foo/bar/;
+
+which is equivalent to
+
+ #!/usr/bin/perl
+ $extension = '.orig';
+ LINE: while (<>) {
+ if ($ARGV ne $oldargv) {
+ if ($extension !~ /\*/) {
+ $backup = $ARGV . $extension;
+ }
+ else {
+ ($backup = $extension) =~ s/\*/$ARGV/g;
+ }
+ rename($ARGV, $backup);
+ open(ARGVOUT, ">$ARGV");
+ select(ARGVOUT);
+ $oldargv = $ARGV;
+ }
+ s/foo/bar/;
+ }
+ continue {
+ print; # this prints to original filename
+ }
+ select(STDOUT);
+
+except that the B<-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...
+or
+ $ perl -p -i'.orig' -e 1 file1 file2 file3...
+
+You can use C<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 L<perlfunc/eof>).
+
+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 B<-i>,
+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?>.
+
+You cannot use B<-i> to create directories or to strip extensions from
+files.
+
+Perl does not expand C<~> 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 B<-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 B<-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.
+
+=item B<-I>I<directory>
+X<-I> X<@INC>
+
+Directories specified by B<-I> are prepended to the search path for
+modules (C<@INC>), and also tells the C preprocessor where to search for
+include files. The C preprocessor is invoked with B<-P>; by default it
+searches /usr/include and /usr/lib/perl.
+
+=item B<-l>[I<octnum>]
+X<-l> X<$/> X<$\>
+
+enables automatic line-ending processing. It has two separate
+effects. First, it automatically chomps C<$/> (the input record
+separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
+(the output record separator) to have the value of I<octnum> so
+that any print statements will have that separator added back on.
+If I<octnum> is omitted, sets C<$\> to the current value of
+C<$/>. For instance, to trim lines to 80 columns:
+
+ perl -lpe 'substr($_, 80) = ""'
+
+Note that the assignment C<$\ = $/> is done when the switch is processed,
+so the input record separator can be different than the output record
+separator if the B<-l> switch is followed by a B<-0> switch:
+
+ gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
+
+This sets C<$\> to newline and then sets C<$/> to the null character.
+
+=item B<-m>[B<->]I<module>
+X<-m> X<-M>
+
+=item B<-M>[B<->]I<module>
+
+=item B<-M>[B<->]I<'module ...'>
+
+=item B<-[mM]>[B<->]I<module=arg[,arg]...>
+
+B<-m>I<module> executes C<use> I<module> C<();> before executing your
+program.
+
+B<-M>I<module> executes C<use> I<module> C<;> before executing your
+program. You can use quotes to add extra code after the module name,
+e.g., C<'-Mmodule qw(foo bar)'>.
+
+If the first character after the B<-M> or B<-m> is a dash (C<->)
+then the 'use' is replaced with 'no'.
+
+A little builtin syntactic sugar means you can also say
+B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
+C<'-Mmodule qw(foo bar)'>. This avoids the need to use quotes when
+importing symbols. The actual code generated by B<-Mmodule=foo,bar> is
+C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
+removes the distinction between B<-m> and B<-M>.
+
+A consequence of this is that B<-MFoo=number> never does a version check
+(unless C<Foo::import()> itself is set up to do a version check, which
+could happen for example if Foo inherits from Exporter.)
+
+=item B<-n>
+X<-n>
+
+causes Perl to assume the following loop around your program, which
+makes it iterate over filename arguments somewhat like B<sed -n> or
+B<awk>:
+
+ LINE:
+ while (<>) {
+ ... # your program goes here
+ }
+
+Note that the lines are not printed by default. See B<-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 C<< <> >> passes command line arguments to
+L<perlfunc/open>, which doesn't necessarily interpret them as file names.
+See L<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 B<-exec> switch of B<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 B<-0>.
+
+C<BEGIN> and C<END> blocks may be used to capture control before or after
+the implicit program loop, just as in B<awk>.
+
+=item B<-p>
+X<-p>
+
+causes Perl to assume the following loop around your program, which
+makes it iterate over filename arguments somewhat like B<sed>:
+
+
+ LINE:
+ 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 B<-n> switch. A B<-p>
+overrides a B<-n> switch.
+
+C<BEGIN> and C<END> blocks may be used to capture control before or after
+the implicit loop, just as in B<awk>.
+
+=item B<-P>
+X<-P>
+
+B<NOTE: Use of -P is strongly discouraged because of its inherent
+problems, including poor portability. It is deprecated and will be
+removed in a future version of Perl.>
+
+This option causes your program to be run through the C preprocessor before
+compilation by Perl. Because both comments and B<cpp> directives begin
+with the # character, you should avoid starting comments with any words
+recognized by the C preprocessor such as C<"if">, C<"else">, or C<"define">.
+
+If you're considering using C<-P>, you might also want to look at the
+Filter::cpp module from CPAN.
+
+The problems of -P include, but are not limited to:
+
+=over 10
+
+=item *
+
+The C<#!> line is stripped, so any switches there don't apply.
+
+=item *
+
+A C<-P> on a C<#!> line doesn't work.
+
+=item *
+
+B<All> lines that begin with (whitespace and) a C<#> but
+do not look like cpp commands, are stripped, including anything
+inside Perl strings, regular expressions, and here-docs .
+
+=item *
+
+In some platforms the C preprocessor knows too much: it knows about
+the C++ -style until-end-of-line comments starting with C<"//">.
+This will cause problems with common Perl constructs like
+
+ s/foo//;
+
+because after -P this will became illegal code
+
+ s/foo
+
+The workaround is to use some other quoting separator than C<"/">,
+like for example C<"!">:
+
+ s!foo!!;
+
+
+
+=item *
+
+It requires not only a working C preprocessor but also a working
+F<sed>. If not on UNIX, you are probably out of luck on this.
+
+=item *
+
+Script line numbers are not preserved.
+
+=item *
+
+The C<-x> does not work with C<-P>.
+
+=back
+
+=item B<-s>
+X<-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 B<-->). 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 B<-xyz> switch, and "abc"
+if it is invoked with B<-xyz=abc>.
+
+ #!/usr/bin/perl -s
+ if ($xyz) { print "$xyz\n" }
+
+Do note that a switch like B<--help> creates the variable ${-help}, which is not compliant
+with C<strict refs>. Also, when using this option on a script with
+warnings enabled you may get a lot of spurious "used only once" warnings.
+
+=item B<-S>
+X<-S>
+
+makes Perl use the PATH environment variable to search for the
+program (unless the name of the program contains directory separators).
+
+On some platforms, this also makes Perl append suffixes to the
+filename while searching for it. For 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:
+
+ #!/usr/bin/perl
+ eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
+ if $running_under_some_shell;
+
+The system ignores the first line and feeds the program to F</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 B<-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 $running_under_some_shell
+is never true. If the program will be interpreted by csh, you will need
+to replace C<${1+"$@"}> with C<$*>, 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 B<csh>, B<sh>, or Perl, such as the following:
+
+ eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
+ & eval 'exec /usr/bin/perl -wS $0 $argv:q'
+ if $running_under_some_shell;
+
+If the filename supplied contains directory separators (i.e., is an
+absolute or relative pathname), and if that file is not found,
+platforms that append file extensions will do so and try to look
+for the file with those extensions added, one by one.
+
+On DOS-like platforms, if the program does not contain directory
+separators, it will first be searched for in the current directory
+before being searched for on the PATH. On Unix platforms, the
+program will be searched for strictly on the PATH.
+
+=item B<-t>
+X<-t>
+
+Like B<-T>, but taint checks will issue warnings rather than fatal
+errors. These warnings can be controlled normally with C<no warnings
+qw(taint)>.
+
+B<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 B<-T>.
+
+=item B<-T>
+X<-T>
+
+forces "taint" checks to be turned on so you can test them. Ordinarily
+these checks are done only when running setuid or setgid. It's a
+good idea to turn them on explicitly for programs that run on behalf
+of someone else whom you might not necessarily trust, such as CGI
+programs or any internet servers you might write in Perl. See
+L<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.
+
+=item B<-u>
+X<-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 B<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 B<undump> is platform
+specific and may not be available for a specific port of Perl.
+
+=item B<-U>
+X<-U>
+
+allows Perl to do unsafe operations. Currently the only "unsafe"
+operations are attempting to unlink directories while running as
+superuser, and running setuid programs with fatal taint checks turned
+into warnings. Note that the B<-w> switch (or the C<$^W> variable)
+must be used along with this option to actually I<generate> the
+taint-check warnings.
+
+=item B<-v>
+X<-v>
+
+prints the version and patchlevel of your perl executable.
+
+=item B<-V>
+X<-V>
+
+prints summary of the major perl configuration values and the current
+values of @INC.
+
+=item B<-V:>I<configvar>
+
+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
+ libc='/lib/libc-2.2.4.so';
+ $ perl -V:lib.
+ libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
+ libc='/lib/libc-2.2.4.so';
+ $ perl -V:lib.*
+ libpth='/usr/local/lib /lib /usr/lib';
+ libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
+ lib_ext='.a';
+ libc='/lib/libc-2.2.4.so';
+ libperl='libperl.a';
+ ....
+
+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`
+ goodvfork=false;
+
+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
+
+=item B<-w>
+X<-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 C<$^W> variable. You
+can disable or promote into fatal errors specific warnings using
+C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
+See also L<perldiag> and L<perltrap>. A new, fine-grained warning
+facility is also available if you want to manipulate entire classes
+of warnings; see L<warnings> or L<perllexwarn>.
+
+=item B<-W>
+X<-W>
+
+Enables all warnings regardless of C<no warnings> or C<$^W>.
+See L<perllexwarn>.
+
+=item B<-X>
+X<-X>
+
+Disables all warnings regardless of C<use warnings> or C<$^W>.
+See L<perllexwarn>.
+
+=item B<-x>
+X<-x>
+
+=item B<-x>I<directory>
+
+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 L<perlsyn/"Plain-Old-Comments-(Not!)">)
+
+If a directory name is specified, Perl will switch to that directory
+before running the program. The B<-x> switch controls only the
+disposal of leading garbage. The program must be terminated with
+C<__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 B<-x>
+with no intervening whitespace.
+
+=back
+
+=head1 ENVIRONMENT
+X<perl, environment variables>
+
+=over 12
+
+=item HOME
+X<HOME>
+
+Used if chdir has no argument.
+
+=item LOGDIR
+X<LOGDIR>
+
+Used if chdir has no argument and HOME is not set.
+
+=item PATH
+X<PATH>
+
+Used in executing subprocesses, and in finding the program if B<-S> is
+used.
+
+=item PERL5LIB
+X<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 C<perl
+-V:path_sep>).
+
+When running taint checks (either because the program was running setuid
+or setgid, or the B<-T> or B<-t> switch was specified), neither variable
+is used. The program should instead say:
+
+ use lib "/my/directory";
+
+=item PERL5OPT
+X<PERL5OPT>
+
+Command-line options (switches). Switches in this variable are taken
+as if they were on every Perl command line. Only the B<-[CDIMUdmtw]>
+switches are allowed. When running taint checks (because the program
+was running setuid or setgid, or the B<-T> switch was used), this
+variable is ignored. If PERL5OPT begins with B<-T>, tainting will be
+enabled, and any subsequent options ignored.
+
+=item PERLIO
+X<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. C<:perlio> to
+emphasise their similarity to variable "attributes". But the code that parses
+layer specification strings (which is also used to decode the PERLIO
+environment variable) treats the colon as a separator.
+
+An unset or empty PERLIO is equivalent to the default set of layers for
+your platform, for example C<:unix:perlio> on UNIX-like systems
+and C<:unix:crlf> on Windows and other DOS-like systems.
+
+The list becomes the default for I<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 L<"open pragma"|open> for how to add external
+encodings as defaults.
+
+The layers that it makes sense to include in the PERLIO environment
+variable are briefly summarised below. For more details see L<PerlIO>.
+
+=over 8
+
+=item :bytes
+X<:bytes>
+
+A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
+Unlikely to be useful on its own in the global PERLIO environment variable.
+You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
+
+=item :crlf
+X<: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 I<not> mimic MS-DOS as far as treating of Control-Z
+as being an end-of-file marker.)
+
+=item :mmap
+X<:mmap>
+
+A layer which implements "reading" of files by using C<mmap()> to
+make (whole) file appear in the process's address space, and then
+using that as PerlIO's "buffer".
+
+=item :perlio
+X<: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 C<:unix>).
+
+=item :pop
+X<:pop>
+
+An experimental pseudolayer that removes the topmost layer.
+Use with the same care as is reserved for nitroglycerin.
+
+=item :raw
+X<:raw>
+
+A pseudolayer that manipulates other layers. Applying the C<:raw>
+layer is equivalent to calling C<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 C<:raw> is I<not>
+just the inverse of C<:crlf> - other layers which would affect the
+binary nature of the stream are also removed or disabled.
+
+=item :stdio
+X<:stdio>
+
+This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
+library calls. The layer provides both buffering and IO.
+Note that C<:stdio> layer does I<not> do CRLF translation even if that
+is platforms normal behaviour. You will need a C<:crlf> layer above it
+to do that.
+
+=item :unix
+X<:unix>
+
+Low level layer which calls C<read>, C<write> and C<lseek> etc.
+
+=item :utf8
+X<: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 C<:encoding(utf8)> is
+the best option when reading UTF-8 encoded data.
+
+=item :win32
+X<:win32>
+
+On Win32 platforms this I<experimental> layer uses native "handle" IO
+rather than unix-like numeric file descriptor layer. Known to be
+buggy in this release.
+
+=back
+
+On all platforms the default set of layers should give acceptable results.
+
+For UNIX platforms that will equivalent of "unix perlio" or "stdio".
+Configure is setup to prefer "stdio" implementation if system's library
+provides for fast access to the buffer, otherwise it uses the "unix perlio"
+implementation.
+
+On Win32 the default in this release is "unix crlf". Win32's "stdio"
+has a number of bugs/mis-features for perl IO which are somewhat
+C compiler vendor/version dependent. Using our own C<crlf> layer as
+the buffer avoids those issues and makes things more uniform.
+The C<crlf> layer provides CRLF to/from "\n" conversion as well as
+buffering.
+
+This release uses C<unix> as the bottom layer on Win32 and so still uses C
+compiler's numeric file descriptor routines. There is an experimental native
+C<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.
+
+=item PERLIO_DEBUG
+X<PERLIO_DEBUG>
+
+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 B<-T>.
+
+=item PERLLIB
+X<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.
+
+=item PERL5DB
+X<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 B<-d> switch.
+
+=item PERL5DB_THREADED
+X<PERL5DB_THREADED>
+
+If set to a true value, indicates to the debugger that the code being
+debugged uses threads.
+
+=item PERL5SHELL (specific to the Win32 port)
+X<PERL5SHELL>
+
+May be set to an alternative shell that perl must use internally for
+executing "backtick" commands or system(). Default is C<cmd.exe /x/d/c>
+on WindowsNT and C<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 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) C<$ENV{PERL5SHELL}> when running
+in taint mode under Windows.
+
+=item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
+X<PERL_ALLOW_NON_IFS_LSP>
+
+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 Windows 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).
+
+=item PERL_DEBUG_MSTATS
+X<PERL_DEBUG_MSTATS>
+
+Relevant only if perl is compiled with the malloc included with the perl
+distribution (that is, if C<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.
+
+=item PERL_DESTRUCT_LEVEL
+X<PERL_DESTRUCT_LEVEL>
+
+Relevant only if your perl executable was built with B<-DDEBUGGING>,
+this controls the behavior of global destruction of objects and other
+references. See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
+
+=item PERL_DL_NONLAZY
+X<PERL_DL_NONLAZY>
+
+Set to one to have perl resolve B<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.
+
+=item PERL_ENCODING
+X<PERL_ENCODING>
+
+If using the C<encoding> pragma without an explicit encoding name, the
+PERL_ENCODING environment variable is consulted for an encoding name.
+
+=item PERL_HASH_SEED
+X<PERL_HASH_SEED>
+
+(Since Perl 5.8.1.) Used to randomise perl's internal hash function.
+To emulate the pre-5.8.1 behaviour, set to an integer (zero means
+exactly the same order as 5.8.0). "Pre-5.8.1" means, among other
+things, that hash keys will 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 insertion, 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 C<-DUSE_HASH_SEED_EXPLICIT>, the default
+behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
+
+If PERL_HASH_SEED is unset or set to a non-numeric string, perl uses
+the pseudorandom seed supplied by the operating system and libraries.
+
+B<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 L<perlsec/"Algorithmic Complexity Attacks"> and
+L</PERL_HASH_SEED_DEBUG> for more information.
+
+=item PERL_HASH_SEED_DEBUG
+X<PERL_HASH_SEED_DEBUG>
+
+(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
+L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
+behavior caused by hash randomization.
+
+B<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 L<perlsec/"Algorithmic Complexity Attacks"> for more information.
+B<Do not disclose the hash seed> to people who don't need to know it.
+See also hash_seed() of L<Hash::Util>.
+
+=item PERL_ROOT (specific to the VMS port)
+X<PERL_ROOT>
+
+A translation concealed rooted logical name that contains perl and the
+logical device for the @INC path on VMS only. Other logical names that
+affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
+SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
+L<perlvms> and in F<README.vms> in the Perl source distribution.
+
+=item PERL_SIGNALS
+X<PERL_SIGNALS>
+
+In Perls 5.8.1 and later. If set to C<unsafe> the pre-Perl-5.8.0
+signals behaviour (immediate but unsafe) is restored. If set to
+C<safe> the safe (or deferred) signals are used.
+See L<perlipc/"Deferred Signals (Safe Signals)">.
+
+=item PERL_UNICODE
+X<PERL_UNICODE>
+
+Equivalent to the B<-C> command-line switch. Note that this is not
+a boolean variable-- setting this to C<"1"> is not the right way to
+"enable Unicode" (whatever that would mean). You can use C<"0"> to
+"disable Unicode", though (or alternatively unset PERL_UNICODE in
+your shell before starting Perl). See the description of the C<-C>
+switch for more information.
+
+=item SYS$LOGIN (specific to the VMS port)
+X<SYS$LOGIN>
+
+Used if chdir has no argument and HOME and LOGDIR are not set.
+
+=back
+
+Perl also has environment variables that control how Perl handles data
+specific to particular natural languages. See L<perllocale>.
+
+Apart from these, Perl uses no other environment variables, except
+to make them available to the program being executed, and to child
+processes. However, programs running setuid would do well to execute
+the following lines before doing anything else, just to keep people
+honest:
+
+ $ENV{PATH} = '/bin:/usr/bin'; # or whatever you need
+ $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
+ delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};