Since you don't post any traffic except the final error, it's difficult to say much about your particular problem. In various programs I've written, a similar problem comes up, where the URI must be extended from relative to absolute with a "base" host or uri derived from the request.

I don't know what your debugging environment is, but as a "general problem solving technique" for LWP::UserAgent, one of the most valuable tools would probably be a proxy in the middle to record all traffic between the application and the server.

There are two methods I frequently use for logging traffic - one is another form of an LWP::UserAgent derived class that logs everything sent to/from the server. Another is an external java tool called "paros" http://www.parosproxy.org. Once you can see all the two-way traffic, it may become more obvious when you need to edit the URI on the fly.