Trying a simple exercise as following: Problem: 3 threads accessing & manipulating a global variable. Conditions: Thread 1: should set the variable to a non-zero value if it sees the previous value as zero. Thread 2: should set the variable to zero if it sees a non-zero value. Thread 3: Simply prints the variable but, only when it changes from zero to non-zero or vice versa. It should not print 2 zeros or, same non-zero number twice. Sub-Conditions: The global variable will be set to 0 at the start. The threads should not step on each other. The non-zero values can be randomly generated My solution so far is: 
use threads;
use threads::shared;
use Thread::Queue;

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

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

    }
}

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

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

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

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My solution hits some racing condition due to which the program hangs. Your suggestions are appreciated

In reply to Threads sharing global variable by Anonymous Monk

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