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X11R7.4 - man page for perlfaq3 (x11r4 section 1)

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

NAME
       perlfaq3 - Programming Tools ($Revision: 10127 $)

DESCRIPTION
       This section of the FAQ answers questions related to programmer tools and programming sup-
       port.

       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 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.

       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 func-
       tionality for control-flow statements and other things. You can get psh at http://source-
       forge.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.

       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 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 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?

       (contributed by brian d foy)

       Before you do anything else, you can help yourself by ensuring that you let Perl tell you
       about problem areas in your code. By turning on warnings and strictures, you can head off
       many problems before they get too big. You can find out more about these in strict and
       warnings.

	       #!/usr/bin/perl
	       use strict;
	       use warnings;

       Beyond that, the simplest debugger is the "print" function. Use it to look at values as
       you run your program:

	       print STDERR "The value is [$value]\n";

       The "Data::Dumper" module can pretty-print Perl data structures:

	       use Data::Dumper qw( Dumper );
	       print STDERR "The hash is " . Dumper( \%hash ) . "\n";

       Perl comes with an interactive debugger, which you can start with the "-d" switch. It's
       fully explained in perldebug.

       If you'd like a graphical user interface and you have Tk, you can use "ptkdb". It's on
       CPAN and available for free.

       If you need something much more sophisticated and controllable, Leon Brocard's Devel::ebug
       (which you can call with the -D switch as -Debug) gives you the programmatic hooks into
       everything you need to write your own (without too much pain and suffering).

       You can also use a commercial debugger such as Affrus (Mac OS X), Komodo from Activestate
       (Windows and Mac OS X), or EPIC (most platforms).

       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 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 refor-
       mat.  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 program-
       mable 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 .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.

       Is there a ctags for Perl?

       (contributed by brian d foy)

       Ctags uses an index to quickly find things in source code, and many popular editors sup-
       port ctags for several different languages, including Perl.

       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 phi-
       losophy of several small tools that each do one thing and do it well.  It's like a carpen-
       ter's toolbox.

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

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

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

       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.

       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.

       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.

       OptiPerl
	   http://www.optiperl.com/

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

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

	   PerlBuidler is an integrated development 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.

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

	   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 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 inter-
       ested 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://ptkdb.sourceforge.net/ ) is a Perl/tk based
       debugger that acts as a development environment of sorts.  Perl Composer ( http://perlcom-
       poser.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

       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 envi-
	   ronment ( http://www.research.att.com/sw/tools/uwin/ )

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

       Zsh 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 Cyg-
       win, MKS, and U/WIN all contain (in addition to the shells) a comprehensive set of stan-
       dard 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).

       Affrus
	   is a full Perl development environment 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 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.bare-
	   bones.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 sup-
       port for the Perl debugger built in.  These should come with the standard Emacs 19 distri-
       bution.

       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 "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
       http://www.cpan.org/authors/Tom_Christiansen/scripts/rep.gz ; this program repeats a com-
       mand and updates the screen as needed, rendering rep ps axu similar to top.

       How can I write a GUI (X, Tk, Gtk, etc.) in Perl?

       (contributed by Ben Morrow)

       There are a number of modules which let you write GUIs in Perl. Most GUI toolkits have a
       perl interface: an incomplete list follows.

       Tk  This works under Unix and Windows, and the current version doesn't look half as bad
	   under Windows as it used to. Some of the gui elements still don't 'feel' quite right,
	   though. The interface is very natural and 'perlish', making it easy to use in small
	   scripts that just need a simple gui. It hasn't been updated in a while.

       Wx  This is a Perl binding for the cross-platform wxWidgets toolkit <http://www.wxwid-
	   gets.org>. It works under Unix, Win32 and Mac OS X, using native widgets (Gtk under
	   Unix). The interface follows the C++ interface closely, but the documentation is a
	   little sparse for someone who doesn't know the library, mostly just referring you to
	   the C++ documentation.

       Gtk and Gtk2
	   These are Perl bindings for the Gtk toolkit <http://www.gtk.org>. The interface
	   changed significantly between versions 1 and 2 so they have separate Perl modules. It
	   runs under Unix, Win32 and Mac OS X (currently it requires an X server on Mac OS, but
	   a 'native' port is underway), and the widgets look the same on every plaform: i.e.,
	   they don't match the native widgets. As with Wx, the Perl bindings follow the C API
	   closely, and the documentation requires you to read the C documentation to understand
	   it.

       Win32::GUI
	   This provides access to most of the Win32 GUI widgets from Perl.  Obviously, it only
	   runs under Win32, and uses native widgets. The Perl interface doesn't really follow
	   the C interface: it's been made more Perlish, and the documentation is pretty good.
	   More advanced stuff may require familiarity with the C Win32 APIs, or reference to
	   MSDN.

       CamelBones
	   CamelBones <http://camelbones.sourceforge.net> is a Perl interface to Mac OS X's Cocoa
	   GUI toolkit, and as such can be used to produce native GUIs on Mac OS X. It's not on
	   CPAN, as it requires frameworks that CPAN.pm doesn't know how to install, but instal-
	   lation is via the standard OSX package installer. The Perl API is, again, very close
	   to the ObjC API it's wrapping, and the documentation just tells you how to translate
	   from one to the other.

       Qt  There is a Perl interface to TrollTech's Qt toolkit, but it does not appear to be
	   maintained.

       Athena
	   Sx is an interface to the Athena widget set which comes with X, but again it appears
	   not to be much used nowadays.

       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 dra-
       matic 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 bottle-
       neck 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 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.

       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 work-
       ing 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 INSTALL file in the source
       distribution.  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 con-
	   cretely, 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 creat-
	   ing a copy of all the contents. This requires some judgement, 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) 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 delete().

       On most operating systems, memory allocated to a program can never be returned to the sys-
       tem. 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 something you can or should be wor-
       rying 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 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/ .

       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
       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 view-
       ing the source.	Security through obscurity, the name for hiding your bugs instead of fix-
       ing 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 later in perlfaq3, but the curious might still be able to de-compile
       it. You can try using the native-code compiler described later, 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.

       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/ ) is Perl's analog to Java's JAR.  It's
       freely available and on CPAN ( http://search.cpan.org/dist/PAR/ ).

       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 convert-
       ing perl scripts to executable files.  It targets both Windows and unix platforms.

       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 "ALTER-
       NATE_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 Reg-
       istry 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 program "install-linux.pl" merely by typ-
       ing "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 "#!" script using Wil Sanchez' DropScript utility:
       http://www.wsanchez.net/software/ .

       IMPORTANT!: Whatever you do, PLEASE don't get frustrated, and just throw the perl inter-
       preter 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.

       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 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 differ-
       ent 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 (including Mac OS X)
	   perl -e 'print "Hello world\n"'

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

	   # Mac Classic
	   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 inter-
       preter.	Under Unix, the first two often work. Under DOS, it's entirely possible that nei-
       ther 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.]

       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 perl-
       faq9 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.

       A good book on OO on Perl is the "Object-Oriented Perl" by Damian Conway from Manning Pub-
       lications, or "Intermediate Perl" by Randal Schwartz, brian d foy, 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, 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 exten-
       sion 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 sup-
       port 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 -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;

       What's MakeMaker?

       (contributed by brian d foy)

       The "ExtUtils::MakeMaker" module, better known simply as "MakeMaker", turns a Perl script,
       typically called "Makefile.PL", into a Makefile.  The unix tool "make" uses this file to
       manage dependencies and actions to process and install a Perl distribution.

REVISION
       Revision: $Revision: 10127 $

       Date: $Date: 2007-10-27 21:40:20 +0200 (Sat, 27 Oct 2007) $

       See perlfaq for source control details and availability.

AUTHOR AND COPYRIGHT
       Copyright (c) 1997-2007 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 comment in the code giving credit to
       the FAQ would be courteous but is not required.

perl v5.8.9				    2007-11-17				      PERLFAQ3(1)


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