From the fine manual.

   shutdown
        $pool->shutdown;

       The "shutdown" method waits for all jobs to be executed, remove
+s all
       worker threads, handles any results that still need to be strea
+med,
       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 cal
+ling
       the autoshutdown method.

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

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