If you were running my posted code, you'd be getting 3 numbers on each "At printer ... " line.

1156646.pl | egrep "printer 0 0"

...
At printer 0 0 59
At printer 0 0 0
At printer 0 0 0
At printer 0 0 0
At printer 0 0 0
At printer 0 0 54
At printer 0 0 0
At printer 0 0 0
At printer 0 0 0
At printer 0 0 0
At printer 0 0 88
At printer 0 0 0
At printer 0 0 74
...
[download]

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

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

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

sub set_zero {
    lock $a;
    while (1) {
        if ($a > 0) {
            $a = 0;
            $q->enqueue($a);
          # print "At set_zero: $a\n";
        } else {
            cond_wait($a)
        }
        cond_broadcast($a);
    }
}

sub printer {
    my ( $v_a, $v_b );
    my $cnt = 100000;
    while ( 1 ) {
        ( $v_a, $v_b ) = $q->dequeue( 2 );
        print "At printer $v_a $v_b ",$a, $/;
      # print "At printer $v_a $v_b\n";
        exit unless --$cnt;
    }
}

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

$_->join() for threads->list;
[download]

That code isn't my code. The lock()s are outside of the while loop, mine are inside.

Once again, I'm not sure what point you are trying to make; but since you can't even get the distinction of the difference between my working code and ikegami's broken code, I've lost interest in continuing...

---

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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)

In the absence of evidence, opinion is indistinguishable from prejudice.