in reply to LWP::UserAgent::FramesReady;

First of all, you should use LWP::UserAgent::FramesReady not LWP::UserAgent to construct the $ua.

my $ua = new LWP::UserAgent::FramesReady;

The result is not an HTTP::Response instance, rather a more appropriate HTTP::Response::Tree object since more than one page is fetched (The main page, the frames, sub-frames, etc...). It's pretty straight forward to get all the responses out with @responses = $response_tree->children();.