MirOS Manual: perlfaq3(1)

PERLFAQ3(1)     Perl Programmers Reference Guide      PERLFAQ3(1)


     perlfaq3 - Programming Tools


     This section of the FAQ answers questions related to pro-
     grammer tools and programming support.

     How do I do (anything)?

     Have you looked at CPAN (see 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 perltoc.

     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 immedi-
     ately evaluated.  You can also examine the symbol table, get
     stack backtraces, check variable values, set breakpoints,
     and other operations typically found in symbolic debuggers.

     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/ .

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     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

     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.

     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

             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;

           sub {
             push @files, $File::Find::name
                     if -f $File::Find::name && /\.pm$/


             print join "\n", @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

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     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

     How do I debug my Perl programs?

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

     Have you tried "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 "my", "our", or
     "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 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 awk and C.

     Have you tried the Perl debugger, described in 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.

     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;

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       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.

     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

     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 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 perlstyle,
     you shouldn't need to reformat.  The habit of formatting
     your code as you write it will help prevent bugs.  Your edi-
     tor 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 programm-
     able editors can provide significant assistance.  Tom Chris-
     tiansen and many other VI users  swear by the following set-
     tings in vi and its clones:

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

     Put that in your .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

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     blockdenting-- as it were.  A more complete example, with
     comments, can be found at

     The a2ps
     does lots of things related to generating nicely printed
     output of documents, as does enscript at
     http://people.ssh.fi/mtr/genscript/ .

     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

     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):


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


         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.


         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.

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     Open Perl IDE

         Open Perl IDE is an integrated development environment
         for writing and debugging Perl scripts with
         ActiveState's ActivePerl distribution under Windows


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


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


         From Help Consulting, for Windows.

     Visual Perl

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


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

     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 edit-
     ing 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 specifi-
     cally for programming, such as Textpad (
     http://www.textpad.com/ ) and UltraEdit (
     http://www.ultraedit.com/ ), among others.

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     If you are using MacOS, the same concerns apply.  MacPerl
     (for Classic environments) comes with a simple editor. Popu-
     lar 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

     GNU Emacs



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

     or a vi clone such as



     Vim http://www.vim.org/

     For vi lovers in general, Windows or elsewhere:


     nvi ( http://www.bostic.com/vi/ , available from CPAN in
     src/misc/) is yet another vi clone, unfortunately not avail-
     able for Windows, but in UNIX platforms you might be
     interested in trying it out, firstly because strictly speak-
     ing 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 sup-
     port Perl:



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     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

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

     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/ )

         ftp://ftp.astron.com/pub/tcsh/ , see also

     Zsh ftp://ftp.blarg.net/users/amol/zsh/ , see also

     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 utili-

     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 con-
     trast to the MacPerl Application the MPW Perl tool can make
     use of the MPW Shell itself as an editor (with no 32k

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

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         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 ).

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

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

     Where can I get Perl macros for vi?

     For a complete version of Tom Christiansen's vi configura-
     tion file, see
     , the standard benchmark file for vi emulators.  The file
     runs best with nvi, the current version of vi out of Berke-
     ley, which incidentally can be built with an embedded Perl
     interpreter--see http://www.cpan.org/src/misc/ .

     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 distribu-

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

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

     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
     ; this program repeats a command and updates the screen as
     needed, rendering rep ps axu similar to top.

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     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

     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

     How can I make my Perl program run faster?

     The best way to do this is to come up with a better algo-
     rithm.  This can often make a dramatic difference.  Jon
     Bentley's book Programming Pearls (that's not a misspel-
     ling!)  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 algo-
     rithms instead of microtuning your code, and when all else
     fails consider just buying faster hardware.  You will prob-
     ably 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 sec-
     tions can be written in C (for instance, the PDL module from

     If you're currently linking your perl executable to a shared
     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 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.

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     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 consid-
     erable 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 com-
     piled 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
     INSTALL file in the source distribution.  You can find out
     whether you are using perl's malloc by typing "perl

     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:

     * 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

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

         instead of this:

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                 # 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.

     * 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/;

     * 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;


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

     * 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 pro-
         pagated 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.

     * 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.

     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";

     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

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     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 sys-
     tems (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 some-
     thing you can or should be worrying about much in Perl.

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

     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 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 inter-
     preter 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

     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.


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     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 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

     Some people regard this as a security problem.  If your pro-
     gram 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

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     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 "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 execut-
     able files.  It targets both Windows and unix platforms.

     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.

     How can I get "#!perl" to work on [MS-DOS,NT,...]?

     For OS/2 just use

         extproc perl -S -your_switches

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PERLFAQ3(1)     Perl Programmers Reference Guide      PERLFAQ3(1)

     as the first line in "*.cmd" file ("-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
     "ALTERNATE_SHEBANG" (see the 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 ".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 ".pl" with the inter-
     preter, NT people can use: "SET PATHEXT=%PATHEXT%;.PL" to
     let them run the program "install-linux.pl" merely by typing

     Under "Classic" MacOS, a perl program will have the
     appropriate Creator and Type, so that double-clicking them
     will invoke the MacPerl application. Under Mac OS X, click-
     able apps can be made from any "#!" script using Wil San-
     chez' DropScript utility: http://www.wsanchez.net/software/

     IMPORTANT!: Whatever you do, PLEASE don't get frustrated,
     and just throw the perl interpreter into your cgi-bin direc-
     tory, 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.

     Can I write useful Perl programs on the command line?

     Yes.  Read perlrun for more information.  Some examples fol-
     low. (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'

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         # display reasonable manpath
         echo $PATH | perl -nl -072 -e '

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

     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 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 prob-
     ably 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 char-

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

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

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

     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 com-
     mand line", see the troubleshooting guides and references in
     perlfaq9 or in the CGI MetaFAQ:


     Where can I learn about object-oriented Perl programming?

     A good place to start is perltoot, and you can use perlobj,
     perlboot, perltoot, perltooc, and 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.

     Where can I learn about linking C with Perl?

     If you want to call C from Perl, start with perlxstut, mov-
     ing on to perlxs, xsubpp, and perlguts.  If you want to call
     Perl from C, then read perlembed, perlcall, and 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.

     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 perlbug and send a bug report with
     the output of "make test TEST_VERBOSE=1" along with "perl

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     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 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;


         use diagnostics -verbose;

     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 ExtUtils::MakeMaker.


     Copyright (c) 1997-2006 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 pro-
     grams for fun or for profit as you see fit.  A simple com-
     ment in the code giving credit to the FAQ would be courteous
     but is not required.

perl v5.8.8                2006-06-30                          20

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