while (1) {
    my $completed_pid = waitpid(-1, WNOHANG);
    if ($completed_pid == -1) {
        # No more children -- exit loop
        last;
    }
    if ($completed_pid > 0) {
        print "<parent>: $completed_pid is done; $childcnt running\n";
            $childcnt--;
        sleep 1;
    }
}