shutdown
        $pool->shutdown;

       The "shutdown" method waits for all jobs to be executed, removes all
       worker threads, handles any results that still need to be streamed,
       before it returns.  Call the abort method if you do not want to wait
       until all jobs have been executed.

       It is called automatically when the object is destroyed, unless
       specifically disabled by providing a false value with the
       "autoshutdown" field when creating the pool with new, or by calling
       the autoshutdown method.

   abort
       The "abort" method waits for all worker threads to finish their
       current job, removes all worker threads, before it returns.  Call the
       shutdown method if you want to wait until all jobs have been done.

       You can restart the job handling process after calling "abort" by
       adding workers again.