unless ($pid = fork) {
        unless (fork) {
            exec "what you really wanna do";
            die "no exec";
            # ... or ...
            ## (some_perl_code_here)
            exit 0;
        }
        exit 0;
    }
    waitpid($pid,0);
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-- Randal L. Schwartz, Perl hacker

If you're running under Win32 (which you probably aren't) then one of the best ways of doing this kind of thing is to use the Win32::Process module. You can fire off a task as a DETACHED_PROCESS and then forget about it. Useful if you don't have a decent fork but not even remotely portable.
--
Ugly
The early bird gets the worm, but the second mouse gets the cheese.

e.g.

system("some command &");

I do this to "parallelize" a bunch of running processes that I don't need to communicate with. You don't get the return value of "some command" but you if you don't care about the return value this should do the trick.