perlfaq3 - Programming Tools

NAME
DESCRIPTION

How do I do (anything)?
How can I use Perl interactively?
Is there a Perl shell?
How do I find which modules are installed on my system?
How do I debug my Perl programs?
How do I profile my Perl programs?
How do I cross-reference my Perl programs?
Is there a pretty-printer (formatter) for Perl?
Is there a ctags for Perl?
Is there an IDE or Windows Perl Editor?
Where can I get Perl macros for vi?
Where can I get perl-mode for emacs?
How can I use curses with Perl?
How can I use X or Tk with Perl?
How can I make my Perl program run faster?
How can I make my Perl program take less memory?
Is it safe to return a reference to local or lexical data?
How can I free an array or hash so my program shrinks?
How can I make my CGI script more efficient?
How can I hide the source for my Perl program?
How can I compile my Perl program into byte code or C?
How can I get #!perl to work on [MS-DOS,NT,...]?
Can I write useful Perl programs on the command line?
Why don't Perl one-liners work on my DOS/Mac/VMS system?
Where can I learn about CGI or Web programming in Perl?
Where can I learn about object-oriented Perl programming?
Where can I learn about linking C with Perl?
I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong?
When I tried to run my script, I got this message. What does it mean?
What's MakeMaker?

REVISION
AUTHOR AND COPYRIGHT

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

Have you looked at CPAN (see the perlfaq2 manpage)? 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:

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

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:

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 functionality for control-flow statements and other things. You can get psh at http://sourceforge.net/projects/psh/ .

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

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

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).

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

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

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).

How do I debug my Perl programs?

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 the strict manpage and the warnings manpage.

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

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

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 sophisicated 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.

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

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.

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

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

Is there a ctags for Perl?

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

Is there an IDE or Windows Perl Editor?

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):

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.

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.

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

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

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

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

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

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?

Where can I get perl-mode for emacs?

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

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

Note that the perl-mode of emacs will have fits with "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 command and updates the screen as needed, rendering rep ps axu similar to top.

How can I use X or Tk with Perl?

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

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 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 working with specialist data structures (matrices, for instance) modules that implement these in C may use less memory than equivalent Perl modules.

Another thing to try is learning whether your Perl was compiled with the system malloc or with Perl's builtin malloc. Whichever one it is, try using the other one and see whether this makes a difference. Information about malloc is in the 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: