I forgot to place the lock back inside the while loop. Sorry about that. The + 1 is necessary to not have double zeros in the output.

sub set_positive {
    while (1) {
        lock $a;
        if ($a == 0) {
            $a = int(rand() * 100) + 1;
            $q->enqueue($a);
            print "At set_positive: $a\n";
        } else {
            cond_wait($a)
        }
        cond_broadcast($a);
    }
}
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