sleep 0.001 while $Q->pending;

== As long as there is something in the queue, do nothing

    for( 1 .. NTHREADS ) {
        $Q->enqueue( scalar <FILE> );
        ...
       }
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== read exactly NTHREADS (i.e. 30) lines from the file and put them in the queue.

And then we are back to doing nothing as long as there is something in the queue

As you can see the call to 'pending' stops the loop from reading more than the NTHREADS lines it is allowed after the queue was empty

The join at the end is just to wait for the threads to exit after they have been signaled to exit. Before that the 'undef's pushed on the queue make sure that all the threads get such a signal. By putting NTHREADS of 'undef's into the queue it is made sure that every threads gets its STOP signal