Don't use a callback then. If you don't use a callback, the response accumulates into the response record. Instead, subclass LWP::Parallel::UserAgent replacing the "on_return" null call with your own:

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

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

           The purpose of $pua->on_return is mainly to provide message
+s when a
           request returns. However, you can also re-register follow-u
+p
           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 meth
+od.
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