use threads;
use threads::shared;
use Thread::Queue;

my $a :shared = 0;
my $q = Thread::Queue->new();

sub set_positive {
    lock $a;
    while (1) {
        cond_wait($a) while $a > 0;

        $a = int(rand() * 100) + 1;
        cond_signal($a);

        $q->enqueue($a);
    }
}

sub set_zero {
    lock $a;
    while (1) {
        cond_wait($a) while $a == 0;

        $a = 0;
        cond_signal($a);

        $q->enqueue($a);
    }
}

sub printer {
    while (1) {
        my ($v_a, $v_b) = $q->dequeue(2);
        print "At printer $v_a $v_b\n";
    }
}

threads->create($_) for qw( set_positive set_zero printer);
$_->join for threads->list;