If you fork() without ever waiting on your children, you will accumulate zombies:

    $SIG{CHLD} = sub { wait };

There's also the double-fork trick (error checking on fork() returns omitted);

    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);

See also the perlipc manpage for more examples of forking and reaping moribund children.

Note that if your forked child inherits system file descriptors like STDIN and STDOUT that are actually connected by a pipe or socket, even if you exit, the remote server (such as, say, httpd or rsh) won't think you're done. You should reopen those to /dev/null if it's any issue.