$ua->on_return ( $request, $response, $entry )
           This method should be overridden in an (otherwise empty) subclass
           in order to present customized messages for each request returned.
           If a callback function was registered with this request, this call-
           back function is called before $pua->on_return.

           Please note that while $pua->on_return is a method (which should be
           overridden in a subclass), a callback function is NOT a method, and
           does not have $self as its first parameter. (See more on callbacks
           below)

           The purpose of $pua->on_return is mainly to provide messages when a
           request returns. However, you can also re-register follow-up
           requests in case you need them.

           If you need specialized follow-up requests depending on the request
           that just returend, use a callback function instead (which can be
           different for each request registered). Otherwise you might end up
           writing a HUGE if..elsif..else.. branch in this global method.