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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 B<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), a Perl executable compiled with sfio
(see the F<INSTALL> file in the distribution) and the mod_fastcgi
module (available from http://www.fastcgi.com/) each of your perl
scripts 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 scripts, so investigate them with
care.

See http://www.perl.com/CPAN/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/ .

A non-free, commerical product, ``The Velocity Engine for Perl'',
(http://www.binevolve.com/ or http://www.binevolve.com/bine/vep) might
also be worth looking at.  It will allow you to increase the performance
of your perl scripts, upto 25 times faster than normal CGI perl by
running in persistent perl mode, or 4 to 5 times faster without any
modification to your existing CGI scripts. Fully functional evaluation
copies are available from the web site.

=head2 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 I<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 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 (Filter::* from CPAN),
but crackers might be able to decrypt it.  You can try using the byte
code compiler and interpreter described below, but crackers might be
able to de-compile it.  You can try using the native-code compiler
described below, but crackers might be able to disassemble it.  These
pose varying degrees of difficulty to people wanting to get at your
code, but none can definitively conceal it (this is true of every
language, not just Perl).

If you're concerned about people profiting from your code, then the
bottom line is that nothing but a restrictive licence 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 licence's wording will stand up in court.

=head2 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 (like 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 I<libperl.so> library and linking against that.  See the
F<INSTALL> podfile in the perl source distribution for details.  If
you link your main perl binary with this, it will make it miniscule.
For example, on one author's system, F</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 will usually hurt
all of those.  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 I<libperl.so>), you'll probably want a complete
Perl install anyway.

=head2 How can I get C<#!perl> to work on [MS-DOS,NT,...]?

For OS/2 just use

    extproc perl -S -your_switches

as the first line in C<*.cmd> file (C<-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 C<ALTERNATIVE_SHEBANG> (see the
F<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 C<.pl> extension with the
perl interpreter.  If you install another port (Gurusaramy Sarathy's
is the recommended Win95/NT port), or (eventually) build your own
Win95/NT Perl using WinGCC, then you'll have to modify the Registry
yourself.

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

I<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 scripts working for a web server.  This is an EXTREMELY big
security risk.  Take the time to figure out how to do it correctly.

=head2 Can I write useful perl programs on the command line?

Yes.  Read L<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 entry. :-)

=head2 Why don't perl one-liners work on my DOS/Mac/VMS system?

The problem is usually that the command interpreters on those systems
have rather different ideas about quoting than the Unix shells under
which the one-liners were created.  On some systems, you may have to
change single-quotes to double ones, which you must I<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)

    # VMS
    perl -e "print ""Hello world\n"""

The problem is that none of this is reliable: it depends on the
command interpreter.  Under Unix, the first two often work. Under DOS,
it's entirely possible neither 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.

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.]

=head2 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 these sources:

    WWW Security FAQ
        http://www.w3.org/Security/Faq/

    Web FAQ
        http://www.boutell.com/faq/

    CGI FAQ
        http://www.webthing.com/page.cgi/cgifaq

    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


=head2 Where can I learn about object-oriented Perl programming?

L<perltoot> is a good place to start, and you can use L<perlobj> and
L<perlbot> for reference.  Perltoot didn't come out until the 5.004
release, but you can get a copy (in pod, html, or postscript) from
http://www.perl.com/CPAN/doc/FMTEYEWTK/ .

=head2 Where can I learn about linking C with Perl? [h2xs, xsubpp]

If you want to call C from Perl, start with L<perlxstut>,
moving on to L<perlxs>, L<xsubpp>, and L<perlguts>.  If you want to
call Perl from C, then read L<perlembed>, L<perlcall>, and
L<perlguts>.  Don't forget that you can learn a lot from looking at
how the authors of existing extension modules wrote their code and
solved their problems.

=head2 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 L<perlbug> and send a bugreport with the output of
C<make test TEST_VERBOSE=1> along with C<perl -V>.

=head2 When I tried to run my script, I got this message. What does it
mean?

L<perldiag> has a complete list of perl's error messages and warnings,
with explanatory text.  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;

=head2 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 L<ExtUtils::MakeMaker>.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997, 1998 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 Licence.  For separate distributions of
all or part of this FAQ outside of that, see L<perlfaq>.

Irrespective of its distribution, all code examples here are 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.