use threads;

...

test_threads();

sub test_threads {
   my $start = time;
   my @thrs;

   # must save threads to join later
   push @thrs, threads->create('work') for (1..$num_workers);

   # wait for threads to finish
   $_->join for @thrs;

   # close the shared file handle, flushes buffer
   $iter_o->close();

   printf STDERR "testa done in %0.02f seconds\n", time - $start;
}