perlfaq3



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




NAME

       perlfaq3 - Programming Tools ($Revision: 1.41 $, $Date: 2004/11/03
       22:45:32 $)


DESCRIPTION

       This section of the FAQ answers questions related to programmer 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 perlde-
       bug(1) manpage, on an ‘‘empty’’ program, like this:

           perl -de 42

       Now just type in any legal Perl code, and it will be immediately evalu-
       ated.  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 functional-
       ity for control-flow statements and other things.  You can get psh at
       http://www.focusresearch.com/gregor/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://zoid-
       berg.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.

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

       You can use the ExtUtils::Installed module to show all installed dis-
       tributions, 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, $File::Find::name if -f _ && /\.pm$/ },
                @INC;

               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 documenta-
       tion, 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

       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 ref-
       erences, 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 pro-
       grammers 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;

         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 perl-
       style. 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 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 Chris-
       tiansen 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 .exrc file (replacing the caret characters with con-
       trol 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 docu-
       ments, as does enscript at http://people.ssh.fi/mtr/genscript/ .

       Is there a ctags for Perl?

       Recent versions of ctags do much more than older versions did.  EXUBER-
       ANT CTAGS is available from http://ctags.sourceforge.net/ and does a
       good job of making tags files for perl code.

       There is also a simple one at
       http://www.cpan.org/authors/id/TOMC/scripts/ptags.gz which may do the
       trick.  It can be easy to hack this into what you want.

       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 phi-
       losophy 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):

       Eclipse
           The Eclipse Perl Integration Project integrates Perl editing/debug-
           ging with Eclipse.

           The website for the project is http://e-p-i-c.sf.net/

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

       Open Perl IDE
           ( http://open-perl-ide.sourceforge.net/ ) Open Perl IDE is an inte-
           grated development environment for writing and debugging Perl
           scripts with ActiveState’s ActivePerl distribution under Windows
           95/98/NT/2000.

       OptiPerl
           ( http://www.optiperl.com/ ) is a Windows IDE with simulated CGI
           environment, including debugger and syntax highlighting editor.

       PerlBuilder
           ( http://www.solutionsoft.com/perl.htm ) is an integrated develop-
           ment environment for Windows that supports Perl development.

       visiPerl+
           ( http://helpconsulting.net/visiperl/ ) From Help Consulting, for
           Windows.

       Visual Perl
           ( http://www.activestate.com/Products/Visual_Perl/ ) Visual Perl is
           a Visual Studio.NET plug-in from ActiveState.

       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 per-
       haps 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.

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

       MicroEMACS
           http://www.microemacs.de/

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

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

       or a vi clone such as

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

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

       Vim http://www.vim.org/

       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:

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

       MultiEdit
           http://www.MultiEdit.com/

       SlickEdit
           http://www.slickedit.com/

       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 cre-
       ation.

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

       Bash
           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/ )

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

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

       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 appropri-
       ately 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 Appli-
       cation the MPW Perl tool can make use of the MPW Shell itself as an
       editor (with no 32k limit).

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

       Alpha
           is an editor, written and extensible in Tcl, that nonetheless has
           built in support for several popular markup and programming lan-
           guages 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 editors 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 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/ .

       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 con-
       text-sensitive help, and other nifty things.

       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 mod-
       ule 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, ren-
       dering rep ps axu similar to top.

       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_Inter-
       faces/

       Invaluable for Perl/Tk programming are the Perl/Tk FAQ at http://pha-
       seit.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/%7Eamund-
       son/perl/perltk/toc.html .

       How can I generate simple menus without using CGI or Tk?

       The http://www.cpan.org/authors/id/SKUNZ/perlmenu.v4.0.tar.gz module,
       which is curses-based, can help with this.

       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 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 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 exe-
       cutable, 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.

       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) mod-
       ules 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 INSTALL file in the source distribu-
       tion.  You can find out whether you are using perl’s malloc by typing
       "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:

       * 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.

       * 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;

           or

                   {
                           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 con-
           tents. 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.

       * Tie large variables to disk.
           For "big" data stores (i.e. ones that exceed available memory) con-
           sider 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 swap-
           ping.

       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?

       You usually can’t. 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.

       However, judicious use of my() on your variables will help make sure
       that they go out of scope so that Perl can free up that space for use
       in other parts of your program.  A global variable, of course, never
       goes out of scope, so you can’t get its space automatically reclaimed,
       although undef()ing and/or delete()ing it will achieve the same effect.
       In general, memory allocation and de-allocation isn’t something you can
       or should be worrying about much in Perl, but even this capability
       (preallocation of data types) is in the works.

       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 isnt 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 any-
       thing 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-cate-
       gory/15_World_Wide_Web_HTML_HTTP_CGI/ .

       A non-free, commercial product, ‘‘The Velocity Engine for Perl’’,
       (http://www.binevolve.com/ or http://www.binevolve.com/velocigen/ )
       might also be worth looking at.  It will allow you to increase the per-
       formance of your Perl programs, running programs up to 25 times faster
       than normal CGI Perl when running in persistent Perl mode or 4 to 5
       times faster without any modification to your existing CGI programs.
       Fully functional evaluation copies are available from the web site.

       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 cant 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 crack-
       ers might be able to disassemble it.  These pose varying degrees of
       difficulty to people wanting to get at your code, but none can defini-
       tively 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.

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

       Malcolm Beattie has written a multifunction backend compiler, available
       from CPAN, that can do both these things.  It is included in the
       perl5.005 release, but is still considered experimental.  This means
       it’s fun to play with if you’re a programmer but not really for people
       looking for turn-key solutions.

       Merely compiling into C does not in and of itself guarantee that your
       code will run very much faster.  That’s because except for lucky cases
       where a lot of native type inferencing is possible, the normal Perl
       run-time system is still present and so your program will take just as
       long to run and be just as big.  Most programs save little more than
       compilation time, leaving execution no more than 10-30% faster.  A few
       rare programs actually benefit significantly (even running several
       times faster), but this takes some tweaking of your code.

       You’ll probably be astonished to learn that the current version of the
       compiler generates a compiled form of your script whose executable is
       just as big as the original perl executable, and then some.  That’s
       because as currently written, all programs are prepared for a full
       eval() statement.  You can tremendously reduce this cost by building a
       shared libperl.so library and linking against that.  See the INSTALL
       podfile in the Perl source distribution for details.  If you link your
       main perl binary with this, it will make it minuscule.  For example, on
       one author’s system, /usr/bin/perl is only 11k in size!

       In general, the compiler will do nothing to make a Perl program
       smaller, faster, more portable, or more secure.  In fact, it can make
       your situation worse.  The executable will be bigger, your VM system
       may take longer to load the whole thing, the binary is fragile and hard
       to fix, and compilation never stopped software piracy in the form of
       crackers, viruses, or bootleggers.  The real advantage of the compiler
       is merely packaging, and once you see the size of what it makes (well,
       unless you use a shared libperl.so), you’ll probably want a complete
       Perl install anyway.

       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 develop-
       ment, 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

       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 Reg-
       istry yourself.  In addition to associating ".pl" with the interpreter,
       NT people can use: "SET PATHEXT=%PATHEXT%;.PL" to let them run the pro-
       gram "install-linux.pl" merely by typing "install-linux".

       Macintosh Perl programs will have the appropriate Creator and Type, so
       that double-clicking them will invoke the Perl application.

       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 secu-
       rity 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 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. :-)

       Why dont 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 com-
       mand 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.]

       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 refer-
       ences in perlfaq9 or in the CGI MetaFAQ:

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

       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.  (If you are using
       really old Perl, you may not have all of these, try http://www.perl-
       doc.com/ , but consider upgrading your perl.)

       A good book on OO on Perl is the "Object-Oriented Perl" by Damian Con-
       way from Manning Publications, http://www.manning.com/Conway/index.html

       Where can I learn about linking C with Perl? [h2xs, xsubpp]

       If you want to call C from Perl, start with perlxstut, moving 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 mod-
       ules wrote their code and solved their problems.

       Ive read perlembed, perlguts, etc., but I cant 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_VER-
       BOSE=1" along with "perl -V".

       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;

       or

           use diagnostics -verbose;

       Whats 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.


AUTHOR AND COPYRIGHT

       Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.  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.



perl v5.8.6                       2004-11-05                       PERLFAQ3(1)

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