Wrap your head around this. It forks away from the cgi, detaches the child process and execs your command. If you're just running a perl script, you can replace the system call with a function you want to run in the background.

use strict;
use POSIX 'setsid';

my $command = q{perl -e 'sleep 30'};

if (my $pid = fork) {
    exit;
} elsif (defined $pid) {
    open STDIN, '/dev/null';
    open STDOUT, '>/dev/null';
    open STDERR, '>/dev/null';

    setsid();

    unless (exec $command) {
        die "Couldn't run process!";
    }

} else {
    die "Cannot fork: $!";
}
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