my $pid;

    ...

    if (!stat($png)) {

            $SIG{'CHLD'} = "wait_for_child";
            $pid = fork();
            if ($pid) {
                    "Creating thumbnail for: $pdf (pid $pid)\n";

            } else {
                    create_thumbnail($pdf,$png);
                    exit(0);
            }
    }

    sub wait_for_child {
            print "Waiting on $pid...";
            sleep(1);
    }