while( my($returned_value) = $queue->dequeue ) {
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You forget that $queue->dequeue always returns an array with at least 1 element. So the condition being checked is always 1 or higher. Something like:

while( my ($returned_value) = $queue->dequeue ) {
    last unless defined $returned_value;
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Liz

I thought that enqueuing an undef would have stopped that while loop