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sub worker_thread
...
{
  $thread -> join();
}

  my $thread_exclusion = Thread::Semaphore -> new ( -1 * $nthreads + 1
+ );

  $thread_exclusion -> up();

sub close_queues
{
  $exclusion_lock -> down();
  $process_q -> end(); 
}

$SIG{'ALRM'} = \&close_queues();
alarm ( 60 );