I think Poco::Timer can dead lock in this scenario:
if you create a timer and you start on a thread pool that has finished his threads you'll receive a NoThreadAvailableException. The exception can suggest you to not call stop() function because the thread is not running, anyway if you do not call stop() the destructor call it for you and you'll wait forever on _done.wait(); because nobody has set the event. probably we should set the event on constructor and reset in ::run() function.
Now I try to figure out a walk around without modify source code if I can