I usually use modules for this purpose. (See perlmod for groovy details.) My main CGI is pretty simple:

#!/usr/bin/perl -wT

use strict;
use SuperModule;
use CGI;

my $q = CGI->new();

# magic here
my %actions = (
    login => \&do_login,
    post => \&do_post,
    pyro => \&set_someone_on_fire
);

my $action = $q->param('action');
if (defined $actions{$action}) {
    $actions{$action}->($q);
} else {
    default_page($q);
}

sub do_login {
    # check login parameters here
    my $q = shift;
    SuperModule::login($q);    # or whatever
}
[download]

...and so forth. It's a layer of abstraction, to be sure, but if you want to put the whole script into your httpd.conf file, it makes mod_perl'ing it a lot easier.