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The undump program was an old attempt to speed up your 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 rele...
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 saving...
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 fil...
Is it unsafe to return a pointer to local data?
No, Perl's garbage collection system takes care of this.
    sub makeone {
        my @a = ( 1 .. 10 );
        return \@a;
    }
    for $i ( 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 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, FreeBSD and Linux) allegedly reclaim large chunks...
We've had reports that on Linux (Redhat 5.1) on Intel, undef $scalar will return memory to the system, while on Solaris 2.6 it won't. In general, try it yourself and see.
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 spa...
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 m...
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 g...
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.perl.com/CPAN/modules/by-category/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 performance of your Perl programs, running program...
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 wit...
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 th...
You can try using encryption via source filters (Filter::* from CPAN), 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 i...
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 proprie...
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 b...
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 yo...
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...
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 bina...
How can I compile Perl into Java?
You can also integrate Java and Perl with the Perl Resource Kit from O'Reilly and Associates. See http://www.oreilly.com/catalog/prkunix/ .
Perl 5.6 comes with Java Perl Lingo, or JPL. JPL, still in development, allows Perl code to be called from Java. See jpl/README in the Perl source tree.
How can I get #!perl to work on [MS-DOS,NT,...]?
For OS/2 just use
    extproc perl -S -your_switches
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 ALTERNATIVE_SHEBANG (see the INSTALL 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...
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 security risk. Take the time to figure out how...
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 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 ...
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)
    # VMS
    perl -e "print ""Hello world\n"""
The problem is that none of these examples are reliable: they depend on the command interpreter. Under Unix, the first two often work. Under DOS, it's entirely possible that neither works. If 4DOS was the command shell, you'd probably have better luc...
  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, pure and simple. Sucks to be away from Unix, huh? :-)
[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...
    WWW Security FAQ
        http://www.w3.org/Security/Faq/
    Web FAQ
        http://www.boutell.com/faq/
    CGI FAQ
        http://www.webthing.com/tutorials/cgifaq.html
    HTTP Spec
        http://www.w3.org/pub/WWW/Protocols/HTTP/
    HTML Spec
        http://www.w3.org/TR/REC-html40/
        http://www.w3.org/pub/WWW/MarkUp/
    CGI Spec
        http://www.w3.org/CGI/
    CGI Security FAQ
        http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt
Where can I learn about object-oriented Perl programming?
A good place to start is perltoot, and you can use perlobj, perlboot, and perlbot for reference. Perltoot didn't come out until the 5.004 release; you can get a copy (in pod, html, or postscript) from http://www.perl.com/CPAN/doc/FMTEYEWTK/ .
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 e...
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?
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) 1997-1999 Tom Christiansen and Nathan Torkington. All rights reserved.
When included as an integrated part of the Standard Distribution of Perl or of its documentation (printed or otherwise), this works is covered under Perl's Artistic License. For separate distributions of all or part of this FAQ outside of that, see p...
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 g...