sub thread_do
{
    # while not all urls are checked
    while (!$DONE) {
        # get new url from boss thread
        my $url = $task_q->dequeue();
        # check url
        my $ua = LWP::UserAgent->new(timeout => 3);
        my $res = $ua->request(HEAD $url);
        # return result to the boss thread
        $result_q->enqueue(
            "$tid;$url;" 
            . $res->code() 
            . ";" 
            . $res->message() 
            . ";"
        );
    }