use Parallel::ForkManager;

...

    my $pm = new Parallel::ForkManager(10);   # max 10 processes simultaneously

    foreach my $job_id (keys %$results) {
        $pm->start and next; # do the fork

        system("perl -w script.pl $job_id");

        $pm->finish; # do the exit in the child process
    }
    $pm->wait_all_children;