There's no need to lock the queue handle; and you need to broadcast on both branches otherwise you've a race condition that is responsible for your hangs; and the sleeps are not useful.

Try this:

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

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

sub set_positive {
    while (1) {
        lock $a;
        if ($a == 0) {
            $a = 1+int(rand() * 100); ## Modified 10/03/2016 15:57 in 
+accordance with 1156654 & 1157305
            $q->enqueue($a);
            print "At set_positive: $a\n";
        } else {
            cond_wait($a)
        }
        cond_broadcast($a);
    }
}

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

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

my @threads = map threads->create($_), qw( set_positive set_zero print
+er);
$_->join for @threads;
[download]

Outputs:

C:\test>1156640
At set_positive: 11
At set_zero: 0
At set_positive: 98
At set_zero: 0
At set_positive: 2
At set_zero: 0
At printer 11 0 0
At printer 98 0 94
At printer 2 0 94
At set_positive: 94
At set_zero: 0
At set_positive: 61
At printer 94 0 0
At set_zero: 0
At printer 61 0 0
At set_positive: 13
At set_zero: 0
At printer 13 0 0
At set_positive: 13
At set_zero: 0
At printer 13 0 0
At set_positive: 28
At set_zero: 0
At printer 28 0 0
At set_positive: 18
At set_zero: 0
At printer 18 0 0
At set_positive: 59
At set_zero: 0
At printer 59 0 0
At set_positive: 15
At set_zero: 0
At printer 15 0 0
At set_positive: 31
At set_zero: 0
At set_positive: 22
At printer 31 0 0
At printer 22 0 0
At set_zero: 0
At set_positive: 19
At set_zero: 0
At printer 19 0 0
At set_positive: 2
At set_zero: 0
At printer 2 0 0
At set_positive: 22
At set_zero: 0
[download]