Notes in perlstyle about POD formatting, by Sébastien

[p5sagit/p5-mst-13.2.git] / pod / perlfaq3.pod

=head1 NAME

perlfaq3 - Programming Tools ($Revision: 1.52 $, $Date: 2005/10/13 19:43:13 $)

=head1 DESCRIPTION

This section of the FAQ answers questions related to programmer tools
and programming support.

=head2 How do I do (anything)?

Have you looked at CPAN (see L<perlfaq2>)?  The chances are that
someone has already written a module that can solve your problem.
Have you read the appropriate manpages?  Here's a brief index:

        Basics          perldata, perlvar, perlsyn, perlop, perlsub
        Execution       perlrun, perldebug
        Functions       perlfunc
        Objects         perlref, perlmod, perlobj, perltie
        Data Structures perlref, perllol, perldsc
        Modules         perlmod, perlmodlib, perlsub
        Regexes         perlre, perlfunc, perlop, perllocale
        Moving to perl5 perltrap, perl
        Linking w/C     perlxstut, perlxs, perlcall, perlguts, perlembed
        Various         http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz
                        (not a man-page but still useful, a collection
                         of various essays on Perl techniques)

A crude table of contents for the Perl manpage set is found in L<perltoc>.

=head2 How can I use Perl interactively?

The typical approach uses the Perl debugger, described in the
perldebug(1) manpage, on an "empty" program, like this:

    perl -de 42

Now just type in any legal Perl code, and it will be immediately
evaluated.  You can also examine the symbol table, get stack
backtraces, check variable values, set breakpoints, and other
operations typically found in symbolic debuggers.

=head2 Is there a Perl shell?

The psh (Perl sh) is currently at version 1.8. The Perl Shell is a
shell that combines the interactive nature of a Unix shell with the
power of Perl. The goal is a full featured shell that behaves as
expected for normal shell activity and uses Perl syntax and
functionality for control-flow statements and other things.
You can get psh at http://sourceforge.net/projects/psh/ .

Zoidberg is a similar project and provides a shell written in perl,
configured in perl and operated in perl. It is intended as a login shell
and development environment. It can be found at http://zoidberg.sf.net/
or your local CPAN mirror.

The Shell.pm module (distributed with Perl) makes Perl try commands
which aren't part of the Perl language as shell commands.  perlsh
from the source distribution is simplistic and uninteresting, but
may still be what you want.

=head2 How do I find which modules are installed on my system?

You can use the ExtUtils::Installed module to show all
installed distributions, although it can take awhile to do
its magic.  The standard library which comes with Perl just
shows up as "Perl" (although you can get those with
Module::CoreList).

        use ExtUtils::Installed;

        my $inst    = ExtUtils::Installed->new();
        my @modules = $inst->modules();

If you want a list of all of the Perl module filenames, you
can use File::Find::Rule.

        use File::Find::Rule;

        my @files = File::Find::Rule->file()->name( '*.pm' )->in( @INC );

If you do not have that module, you can do the same thing
with File::Find which is part of the standard library.

    use File::Find;
    my @files;

    find(
      sub { push @files, $File::Find::name if -f $File::Find::name && /\.pm$/ },
      @INC
    );

    print "$_\n" for @files;

If you simply need to quickly check to see if a module is
available, you can check for its documentation.  If you can
read the documentation the module is most likely installed.
If you cannot read the documentation, the module might not
have any (in rare cases).

        prompt% perldoc Module::Name

You can also try to include the module in a one-liner to see if
perl finds it.

        perl -MModule::Name -e1

=head2 How do I debug my Perl programs?

Have you tried C<use warnings> or used C<-w>?  They enable warnings
to detect dubious practices.

Have you tried C<use strict>?  It prevents you from using symbolic
references, makes you predeclare any subroutines that you call as bare
words, and (probably most importantly) forces you to predeclare your
variables with C<my>, C<our>, or C<use vars>.

Did you check the return values of each and every system call?  The operating
system (and thus Perl) tells you whether they worked, and if not
why.

  open(FH, "> /etc/cantwrite")
    or die "Couldn't write to /etc/cantwrite: $!\n";

Did you read L<perltrap>?  It's full of gotchas for old and new Perl
programmers and even has sections for those of you who are upgrading
from languages like I<awk> and I<C>.

Have you tried the Perl debugger, described in L<perldebug>?  You can
step through your program and see what it's doing and thus work out
why what it's doing isn't what it should be doing.

=head2 How do I profile my Perl programs?

You should get the Devel::DProf module from the standard distribution
(or separately on CPAN) and also use Benchmark.pm from the standard
distribution.  The Benchmark module lets you time specific portions of
your code, while Devel::DProf gives detailed breakdowns of where your
code spends its time.

Here's a sample use of Benchmark:

  use Benchmark;

  @junk = `cat /etc/motd`;
  $count = 10_000;

  timethese($count, {
            'map' => sub { my @a = @junk;
                           map { s/a/b/ } @a;
                           return @a },
            'for' => sub { my @a = @junk;
                           for (@a) { s/a/b/ };
                           return @a },
           });

This is what it prints (on one machine--your results will be dependent
on your hardware, operating system, and the load on your machine):

  Benchmark: timing 10000 iterations of for, map...
         for:  4 secs ( 3.97 usr  0.01 sys =  3.98 cpu)
         map:  6 secs ( 4.97 usr  0.00 sys =  4.97 cpu)

Be aware that a good benchmark is very hard to write.  It only tests the
data you give it and proves little about the differing complexities
of contrasting algorithms.

=head2 How do I cross-reference my Perl programs?

The B::Xref module can be used to generate cross-reference reports
for Perl programs.

    perl -MO=Xref[,OPTIONS] scriptname.plx

=head2 Is there a pretty-printer (formatter) for Perl?

Perltidy is a Perl script which indents and reformats Perl scripts
to make them easier to read by trying to follow the rules of the
L<perlstyle>. If you write Perl scripts, or spend much time reading
them, you will probably find it useful.  It is available at
http://perltidy.sourceforge.net

Of course, if you simply follow the guidelines in L<perlstyle>,
you shouldn't need to reformat.  The habit of formatting your code
as you write it will help prevent bugs.  Your editor can and should
help you with this.  The perl-mode or newer cperl-mode for emacs
can provide remarkable amounts of help with most (but not all)
code, and even less programmable editors can provide significant
assistance.  Tom Christiansen and many other VI users  swear by
the following settings in vi and its clones:

    set ai sw=4
    map! ^O {^M}^[O^T

Put that in your F<.exrc> file (replacing the caret characters
with control characters) and away you go.  In insert mode, ^T is
for indenting, ^D is for undenting, and ^O is for blockdenting--
as it were.  A more complete example, with comments, can be found at
http://www.cpan.org/authors/id/TOMC/scripts/toms.exrc.gz

The a2ps http://www-inf.enst.fr/%7Edemaille/a2ps/black+white.ps.gz does
lots of things related to generating nicely printed output of
documents, as does enscript at http://people.ssh.fi/mtr/genscript/ .

=head2 Is there a ctags for Perl?

(contributed by brian d foy)

Exuberent ctags supports Perl: http://ctags.sourceforge.net/

You might also try pltags: http://www.mscha.com/pltags.zip

=head2 Is there an IDE or Windows Perl Editor?

Perl programs are just plain text, so any editor will do.

If you're on Unix, you already have an IDE--Unix itself.  The UNIX
philosophy is the philosophy of several small tools that each do one
thing and do it well.  It's like a carpenter's toolbox.

If you want an IDE, check the following (in alphabetical order, not
order of preference):

=over 4

=item Eclipse

http://e-p-i-c.sf.net/

The Eclipse Perl Integration Project integrates Perl
editing/debugging with Eclipse.

=item Enginsite

http://www.enginsite.com/

Perl Editor by EngInSite is a complete integrated development
environment (IDE) for creating, testing, and  debugging  Perl scripts;
the tool runs on Windows 9x/NT/2000/XP or later.

=item Komodo

http://www.ActiveState.com/Products/Komodo/

ActiveState's cross-platform (as of October 2004, that's Windows, Linux,
and Solaris), multi-language IDE has Perl support, including a regular expression
debugger and remote debugging.

=item Open Perl IDE

http://open-perl-ide.sourceforge.net/

Open Perl IDE is an integrated development environment for writing
and debugging Perl scripts with ActiveState's ActivePerl distribution
under Windows 95/98/NT/2000.

=item OptiPerl

http://www.optiperl.com/

OptiPerl is a Windows IDE with simulated CGI environment, including
debugger and syntax highlighting editor.

=item PerlBuilder

http://www.solutionsoft.com/perl.htm

PerlBuidler is an integrated development environment for Windows that
supports Perl development.

=item visiPerl+

http://helpconsulting.net/visiperl/

From Help Consulting, for Windows.

=item Visual Perl

http://www.activestate.com/Products/Visual_Perl/

Visual Perl is a Visual Studio.NET plug-in from ActiveState.

=item Zeus

http://www.zeusedit.com/lookmain.html

Zeus for Window is another Win32 multi-language editor/IDE
that comes with support for Perl:

=back

For editors: if you're on Unix you probably have vi or a vi clone
already, and possibly an emacs too, so you may not need to download
anything. In any emacs the cperl-mode (M-x cperl-mode) gives you
perhaps the best available Perl editing mode in any editor.

If you are using Windows, you can use any editor that lets you work
with plain text, such as NotePad or WordPad.  Word processors, such as
Microsoft Word or WordPerfect, typically do not work since they insert
all sorts of behind-the-scenes information, although some allow you to
save files as "Text Only". You can also download text editors designed
specifically for programming, such as Textpad (
http://www.textpad.com/ ) and UltraEdit ( http://www.ultraedit.com/ ),
among others.

If you are using MacOS, the same concerns apply.  MacPerl (for Classic
environments) comes with a simple editor. Popular external editors are
BBEdit ( http://www.bbedit.com/ ) or Alpha (
http://www.his.com/~jguyer/Alpha/Alpha8.html ). MacOS X users can use
Unix editors as well. Neil Bowers (the man behind Geekcruises) has a
list of Mac editors that can handle Perl (
http://www.neilbowers.org/macperleditors.html ).

=over 4

=item GNU Emacs

http://www.gnu.org/software/emacs/windows/ntemacs.html

=item MicroEMACS

http://www.microemacs.de/

=item XEmacs

http://www.xemacs.org/Download/index.html

=item Jed

http://space.mit.edu/~davis/jed/

=back

or a vi clone such as

=over 4

=item Elvis

ftp://ftp.cs.pdx.edu/pub/elvis/ http://www.fh-wedel.de/elvis/

=item Vile

http://dickey.his.com/vile/vile.html

=item Vim

http://www.vim.org/

=back

For vi lovers in general, Windows or elsewhere:

        http://www.thomer.com/thomer/vi/vi.html

nvi ( http://www.bostic.com/vi/ , available from CPAN in src/misc/) is
yet another vi clone, unfortunately not available for Windows, but in
UNIX platforms you might be interested in trying it out, firstly because
strictly speaking it is not a vi clone, it is the real vi, or the new
incarnation of it, and secondly because you can embed Perl inside it
to use Perl as the scripting language.  nvi is not alone in this,
though: at least also vim and vile offer an embedded Perl.

The following are Win32 multilanguage editor/IDESs that support Perl:

=over 4

=item Codewright

http://www.borland.com/codewright/

=item MultiEdit

http://www.MultiEdit.com/

=item SlickEdit

http://www.slickedit.com/

=back

There is also a toyedit Text widget based editor written in Perl
that is distributed with the Tk module on CPAN.  The ptkdb
( http://world.std.com/~aep/ptkdb/ ) is a Perl/tk based debugger that
acts as a development environment of sorts.  Perl Composer
( http://perlcomposer.sourceforge.net/ ) is an IDE for Perl/Tk
GUI creation.

In addition to an editor/IDE you might be interested in a more
powerful shell environment for Win32.  Your options include

=over 4

=item Bash

from the Cygwin package ( http://sources.redhat.com/cygwin/ )

=item Ksh

from the MKS Toolkit ( http://www.mks.com/ ), or the Bourne shell of
the U/WIN environment ( http://www.research.att.com/sw/tools/uwin/ )

=item Tcsh

ftp://ftp.astron.com/pub/tcsh/ , see also
http://www.primate.wisc.edu/software/csh-tcsh-book/

=item Zsh

ftp://ftp.blarg.net/users/amol/zsh/ , see also http://www.zsh.org/

=back

MKS and U/WIN are commercial (U/WIN is free for educational and
research purposes), Cygwin is covered by the GNU Public License (but
that shouldn't matter for Perl use).  The Cygwin, MKS, and U/WIN all
contain (in addition to the shells) a comprehensive set of standard
UNIX toolkit utilities.

If you're transferring text files between Unix and Windows using FTP
be sure to transfer them in ASCII mode so the ends of lines are
appropriately converted.

On Mac OS the MacPerl Application comes with a simple 32k text editor
that behaves like a rudimentary IDE.  In contrast to the MacPerl Application
the MPW Perl tool can make use of the MPW Shell itself as an editor (with
no 32k limit).

=over 4

=item Affrus

is a full Perl development environment with full debugger support
( http://www.latenightsw.com ).

=item Alpha

is an editor, written and extensible in Tcl, that nonetheless has
built in support for several popular markup and programming languages
including Perl and HTML ( http://www.his.com/~jguyer/Alpha/Alpha8.html ).

=item BBEdit and BBEdit Lite

are text editors for Mac OS that have a Perl sensitivity mode
( http://web.barebones.com/ ).


=back

Pepper and Pe are programming language sensitive text editors for Mac
OS X and BeOS respectively ( http://www.hekkelman.com/ ).

=head2 Where can I get Perl macros for vi?

For a complete version of Tom Christiansen's vi configuration file,
see http://www.cpan.org/authors/Tom_Christiansen/scripts/toms.exrc.gz ,
the standard benchmark file for vi emulators.  The file runs best with nvi,
the current version of vi out of Berkeley, which incidentally can be built
with an embedded Perl interpreter--see http://www.cpan.org/src/misc/ .

=head2 Where can I get perl-mode for emacs?

Since Emacs version 19 patchlevel 22 or so, there have been both a
perl-mode.el and support for the Perl debugger built in.  These should
come with the standard Emacs 19 distribution.

In the Perl source directory, you'll find a directory called "emacs",
which contains a cperl-mode that color-codes keywords, provides
context-sensitive help, and other nifty things.

Note that the perl-mode of emacs will have fits with C<"main'foo">
(single quote), and mess up the indentation and highlighting.  You
are probably using C<"main::foo"> in new Perl code anyway, so this
shouldn't be an issue.

=head2 How can I use curses with Perl?

The Curses module from CPAN provides a dynamically loadable object
module interface to a curses library.  A small demo can be found at the
directory http://www.cpan.org/authors/Tom_Christiansen/scripts/rep.gz ;
this program repeats a command and updates the screen as needed, rendering
B<rep ps axu> similar to B<top>.

=head2 How can I use X or Tk with Perl?

Tk is a completely Perl-based, object-oriented interface to the Tk toolkit
that doesn't force you to use Tcl just to get at Tk.  Sx is an interface
to the Athena Widget set.  Both are available from CPAN.  See the
directory http://www.cpan.org/modules/by-category/08_User_Interfaces/

Invaluable for Perl/Tk programming are the Perl/Tk FAQ at
http://phaseit.net/claird/comp.lang.perl.tk/ptkFAQ.html , the Perl/Tk Reference
Guide available at
http://www.cpan.org/authors/Stephen_O_Lidie/ , and the
online manpages at
http://www-users.cs.umn.edu/%7Eamundson/perl/perltk/toc.html .

=head2 How can I make my Perl program run faster?

The best way to do this is to come up with a better algorithm.  This
can often make a dramatic difference.  Jon Bentley's book
I<Programming Pearls> (that's not a misspelling!)  has some good tips
on optimization, too.  Advice on benchmarking boils down to: benchmark
and profile to make sure you're optimizing the right part, look for
better algorithms instead of microtuning your code, and when all else
fails consider just buying faster hardware.  You will probably want to
read the answer to the earlier question "How do I profile my Perl
programs?" if you haven't done so already.

A different approach is to autoload seldom-used Perl code.  See the
AutoSplit and AutoLoader modules in the standard distribution for
that.  Or you could locate the bottleneck and think about writing just
that part in C, the way we used to take bottlenecks in C code and
write them in assembler.  Similar to rewriting in C, modules that have
critical sections can be written in C (for instance, the PDL module
from CPAN).

If you're currently linking your perl executable to a shared
I<libc.so>, you can often gain a 10-25% performance benefit by
rebuilding it to link with a static libc.a instead.  This will make a
bigger perl executable, but your Perl programs (and programmers) may
thank you for it.  See the F<INSTALL> file in the source distribution
for more information.

The undump program was an ancient attempt to speed up Perl program by
storing the already-compiled form to disk.  This is no longer a viable
option, as it only worked on a few architectures, and wasn't a good
solution anyway.

=head2 How can I make my Perl program take less memory?

When it comes to time-space tradeoffs, Perl nearly always prefers to
throw memory at a problem.  Scalars in Perl use more memory than
strings in C, arrays take more than that, and hashes use even more.  While
there's still a lot to be done, recent releases have been addressing
these issues.  For example, as of 5.004, duplicate hash keys are
shared amongst all hashes using them, so require no reallocation.

In some cases, using substr() or vec() to simulate arrays can be
highly beneficial.  For example, an array of a thousand booleans will
take at least 20,000 bytes of space, but it can be turned into one
125-byte bit vector--a considerable memory savings.  The standard
Tie::SubstrHash module can also help for certain types of data
structure.  If you're working with specialist data structures
(matrices, for instance) modules that implement these in C may use
less memory than equivalent Perl modules.

Another thing to try is learning whether your Perl was compiled with
the system malloc or with Perl's builtin malloc.  Whichever one it
is, try using the other one and see whether this makes a difference.
Information about malloc is in the F<INSTALL> file in the source
distribution.  You can find out whether you are using perl's malloc by
typing C<perl -V:usemymalloc>.

Of course, the best way to save memory is to not do anything to waste
it in the first place. Good programming practices can go a long way
toward this:

=over 4

=item * Don't slurp!

Don't read an entire file into memory if you can process it line
by line. Or more concretely, use a loop like this:

        #
        # Good Idea
        #
        while (<FILE>) {
           # ...
        }

instead of this:

        #
        # Bad Idea
        #
        @data = <FILE>;
        foreach (@data) {
            # ...
        }

When the files you're processing are small, it doesn't much matter which
way you do it, but it makes a huge difference when they start getting
larger.

=item * Use map and grep selectively

Remember that both map and grep expect a LIST argument, so doing this:

        @wanted = grep {/pattern/} <FILE>;

will cause the entire file to be slurped. For large files, it's better
to loop:

        while (<FILE>) {
                push(@wanted, $_) if /pattern/;
        }

=item * Avoid unnecessary quotes and stringification

Don't quote large strings unless absolutely necessary:

        my $copy = "$large_string";

makes 2 copies of $large_string (one for $copy and another for the
quotes), whereas

        my $copy = $large_string;

only makes one copy.

Ditto for stringifying large arrays:

        {
                local $, = "\n";
                print @big_array;
        }

is much more memory-efficient than either

        print join "\n", @big_array;

or

        {
                local $" = "\n";
                print "@big_array";
        }


=item * Pass by reference

Pass arrays and hashes by reference, not by value. For one thing, it's
the only way to pass multiple lists or hashes (or both) in a single
call/return. It also avoids creating a copy of all the contents. This
requires some judgment, however, because any changes will be propagated
back to the original data. If you really want to mangle (er, modify) a
copy, you'll have to sacrifice the memory needed to make one.

=item * Tie large variables to disk.

For "big" data stores (i.e. ones that exceed available memory) consider
using one of the DB modules to store it on disk instead of in RAM. This
will incur a penalty in access time, but that's probably better than
causing your hard disk to thrash due to massive swapping.

=back

=head2 Is it safe to return a reference to local or lexical data?

Yes. Perl's garbage collection system takes care of this so
everything works out right.

    sub makeone {
        my @a = ( 1 .. 10 );
        return \@a;
    }

    for ( 1 .. 10 ) {
        push @many, makeone();
    }

    print $many[4][5], "\n";

    print "@many\n";

=head2 How can I free an array or hash so my program shrinks?

(contributed by Michael Carman)

You usually can't. Memory allocated to lexicals (i.e. my() variables)
cannot be reclaimed or reused even if they go out of scope. It is
reserved in case the variables come back into scope. Memory allocated
to global variables can be reused (within your program) by using
undef()ing and/or delete().

On most operating systems, memory allocated to a program can never be
returned to the system. That's why long-running programs sometimes re-
exec themselves. Some operating systems (notably, systems that use
mmap(2) for allocating large chunks of memory) can reclaim memory that
is no longer used, but on such systems, perl must be configured and
compiled to use the OS's malloc, not perl's.

In general, memory allocation and de-allocation isn't something you can
or should be worrying about much in Perl.

See also "How can I make my Perl program take less memory?"

=head2 How can I make my CGI script more efficient?

Beyond the normal measures described to make general Perl programs
faster or smaller, a CGI program has additional issues.  It may be run
several times per second.  Given that each time it runs it will need
to be re-compiled and will often allocate a megabyte or more of system
memory, this can be a killer.  Compiling into C B<isn't going to help
you> because the process start-up overhead is where the bottleneck is.

There are two popular ways to avoid this overhead.  One solution
involves running the Apache HTTP server (available from
http://www.apache.org/ ) with either of the mod_perl or mod_fastcgi
plugin modules.

With mod_perl and the Apache::Registry module (distributed with
mod_perl), httpd will run with an embedded Perl interpreter which
pre-compiles your script and then executes it within the same address
space without forking.  The Apache extension also gives Perl access to
the internal server API, so modules written in Perl can do just about
anything a module written in C can.  For more on mod_perl, see
http://perl.apache.org/

With the FCGI module (from CPAN) and the mod_fastcgi
module (available from http://www.fastcgi.com/ ) each of your Perl
programs becomes a permanent CGI daemon process.

Both of these solutions can have far-reaching effects on your system
and on the way you write your CGI programs, so investigate them with
care.

See http://www.cpan.org/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/ .

=head2 How can I hide the source for my Perl program?

Delete it. :-) Seriously, there are a number of (mostly
unsatisfactory) solutions with varying levels of "security".

First of all, however, you I<can't> take away read permission, because
the source code has to be readable in order to be compiled and
interpreted.  (That doesn't mean that a CGI script's source is
readable by people on the web, though--only by people with access to
the filesystem.)  So you have to leave the permissions at the socially
friendly 0755 level.

Some people regard this as a security problem.  If your program does
insecure things and relies on people not knowing how to exploit those
insecurities, it is not secure.  It is often possible for someone to
determine the insecure things and exploit them without viewing the
source.  Security through obscurity, the name for hiding your bugs
instead of fixing them, is little security indeed.

You can try using encryption via source filters (Starting from Perl
5.8 the Filter::Simple and Filter::Util::Call modules are included in
the standard distribution), but any decent programmer will be able to
decrypt it.  You can try using the byte code compiler and interpreter
described below, but the curious might still be able to de-compile it.
You can try using the native-code compiler described below, but
crackers might be able to disassemble it.  These pose varying degrees
of difficulty to people wanting to get at your code, but none can
definitively conceal it (true of every language, not just Perl).

It is very easy to recover the source of Perl programs.  You simply
feed the program to the perl interpreter and use the modules in
the B:: hierarchy.  The B::Deparse module should be able to
defeat most attempts to hide source.  Again, this is not
unique to Perl.

If you're concerned about people profiting from your code, then the
bottom line is that nothing but a restrictive license will give you
legal security.  License your software and pepper it with threatening
statements like "This is unpublished proprietary software of XYZ Corp.
Your access to it does not give you permission to use it blah blah
blah."  We are not lawyers, of course, so you should see a lawyer if
you want to be sure your license's wording will stand up in court.

=head2 How can I compile my Perl program into byte code or C?

(contributed by brian d foy)

In general, you can't do this.  There are some things that may work
for your situation though.  People usually ask this question
because they want to distribute their works without giving away
the source code, and most solutions trade disk space for convenience.
You probably won't see much of a speed increase either, since most
solutions simply bundle a Perl interpreter in the final product
(but see L<How can I make my Perl program run faster?>).

The Perl Archive Toolkit ( http://par.perl.org/index.cgi ) is Perl's
analog to Java's JAR.  It's freely available and on CPAN (
http://search.cpan.org/dist/PAR/ ).

The B::* namespace, often called "the Perl compiler", but is really a way
for Perl programs to peek at its innards rather than create pre-compiled
versions of your program.  However. the B::Bytecode module can turn your
script  into a bytecode format that could be loaded later by the
ByteLoader module and executed as a regular Perl script.

There are also some commercial products that may work for you, although
you have to buy a license for them.

The Perl Dev Kit ( http://www.activestate.com/Products/Perl_Dev_Kit/ )
from ActiveState can "Turn your Perl programs into ready-to-run
executables for HP-UX, Linux, Solaris and Windows."

Perl2Exe ( http://www.indigostar.com/perl2exe.htm ) is a command line
program for converting perl scripts to executable files.  It targets both
Windows and unix platforms.

=head2 How can I compile Perl into Java?

You can also integrate Java and Perl with the
Perl Resource Kit from O'Reilly Media.  See
http://www.oreilly.com/catalog/prkunix/ .

Perl 5.6 comes with Java Perl Lingo, or JPL.  JPL, still in
development, allows Perl code to be called from Java.  See jpl/README
in the Perl source tree.

=head2 How can I get C<#!perl> to work on [MS-DOS,NT,...]?

For OS/2 just use

    extproc perl -S -your_switches

as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's
"extproc" handling).  For DOS one should first invent a corresponding
batch file and codify it in C<ALTERNATE_SHEBANG> (see the
F<dosish.h> file in the source distribution for more information).

The Win95/NT installation, when using the ActiveState port of Perl,
will modify the Registry to associate the C<.pl> extension with the
perl interpreter.  If you install another port, perhaps even building
your own Win95/NT Perl from the standard sources by using a Windows port
of gcc (e.g., with cygwin or mingw32), then you'll have to modify
the Registry yourself.  In addition to associating C<.pl> with the
interpreter, NT people can use: C<SET PATHEXT=%PATHEXT%;.PL> to let them
run the program C<install-linux.pl> merely by typing C<install-linux>.

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/ .

I<IMPORTANT!>: Whatever you do, PLEASE don't get frustrated, and just
throw the perl interpreter into your cgi-bin directory, in order to
get your programs working for a web server.  This is an EXTREMELY big
security risk.  Take the time to figure out how to do it correctly.

=head2 Can I write useful Perl programs on the command line?

Yes.  Read L<perlrun> for more information.  Some examples follow.
(These assume standard Unix shell quoting rules.)

    # sum first and last fields
    perl -lane 'print $F[0] + $F[-1]' *

    # identify text files
    perl -le 'for(@ARGV) {print if -f && -T _}' *

    # remove (most) comments from C program
    perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c

    # make file a month younger than today, defeating reaper daemons
    perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' *

    # find first unused uid
    perl -le '$i++ while getpwuid($i); print $i'

    # display reasonable manpath
    echo $PATH | perl -nl -072 -e '
        s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}'

OK, the last one was actually an Obfuscated Perl Contest entry. :-)

=head2 Why don't Perl one-liners work on my DOS/Mac/VMS system?

The problem is usually that the command interpreters on those systems
have rather different ideas about quoting than the Unix shells under
which the one-liners were created.  On some systems, you may have to
change single-quotes to double ones, which you must I<NOT> do on Unix
or Plan9 systems.  You might also have to change a single % to a %%.

For example:

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

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

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

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

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

The problem is that none of these examples are reliable: they depend on the
command interpreter.  Under Unix, the first two often work. Under DOS,
it's entirely possible that neither works.  If 4DOS was the command shell,
you'd probably have better luck like this:

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

Under the Mac, 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 Mac's non-ASCII
characters as control characters.

Using qq(), q(), and qx(), instead of "double quotes", 'single
quotes', and `backticks`, may make one-liners easier to write.

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

[Some of this answer was contributed by Kenneth Albanowski.]

=head2 Where can I learn about CGI or Web programming in Perl?

For modules, get the CGI or LWP modules from CPAN.  For textbooks,
see the two especially dedicated to web stuff in the question on
books.  For problems and questions related to the web, like "Why
do I get 500 Errors" or "Why doesn't it run from the browser right
when it runs fine on the command line", see the troubleshooting
guides and references in L<perlfaq9> or in the CGI MetaFAQ:

        http://www.perl.org/CGI_MetaFAQ.html

=head2 Where can I learn about object-oriented Perl programming?

A good place to start is L<perltoot>, and you can use L<perlobj>,
L<perlboot>, L<perltoot>, L<perltooc>, and L<perlbot> for reference.

A good book on OO on Perl is the "Object-Oriented Perl"
by Damian Conway from Manning Publications, or "Learning Perl
References, Objects, & Modules" by Randal Schwartz and Tom
Phoenix from O'Reilly Media.

=head2 Where can I learn about linking C with Perl?

If you want to call C from Perl, start with L<perlxstut>,
moving on to L<perlxs>, L<xsubpp>, and L<perlguts>.  If you want to
call Perl from C, then read L<perlembed>, L<perlcall>, and
L<perlguts>.  Don't forget that you can learn a lot from looking at
how the authors of existing extension modules wrote their code and
solved their problems.

You might not need all the power of XS. The Inline::C module lets
you put C code directly in your Perl source. It handles all the
magic to make it work. You still have to learn at least some of 
the perl API but you won't have to deal with the complexity of the
XS support files.

=head2 I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong?

Download the ExtUtils::Embed kit from CPAN and run `make test'.  If
the tests pass, read the pods again and again and again.  If they
fail, see L<perlbug> and send a bug report with the output of
C<make test TEST_VERBOSE=1> along with C<perl -V>.

=head2 When I tried to run my script, I got this message. What does it mean?

A complete list of Perl's error messages and warnings with explanatory
text can be found in L<perldiag>. You can also use the splain program
(distributed with Perl) to explain the error messages:

    perl program 2>diag.out
    splain [-v] [-p] diag.out

or change your program to explain the messages for you:

    use diagnostics;

or

    use diagnostics -verbose;

=head2 What's MakeMaker?

This module (part of the standard Perl distribution) is designed to
write a Makefile for an extension module from a Makefile.PL.  For more
information, see L<ExtUtils::MakeMaker>.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.

This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.

Irrespective of its distribution, all code examples here are in the public
domain.  You are permitted and encouraged to use this code and any
derivatives thereof in your own programs for fun or for profit as you
see fit.  A simple comment in the code giving credit to the FAQ would
be courteous but is not required.