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   my $manager = new Parallel::ForkManager( 20 );

   $manager->set_max_procs( $newMaximumProcs );

   foreach my $command (@commands) {
      $manager->start and next;
...
      $manager->finish;
   };

   $manager->run_on_start( 
      sub {
...
      }
    );

   $manager->run_on_finish( 
      sub {
...
      }
   );

   $manager->wait_on_finish( 
      sub {
...
      3
   );