interesting .. that 500 error message 'read timeoutt' is a rather odd spelling and I expect it's not coming from LWP but from the target server. Sorry I cannot think of a reason off the top of my head, but here's how I would approach the problem:

• double check the PHP client to ensure you're sending all the info (any missing headers?)
• if you have access to the target server, check its' logs for clues
• use a command line client (like curl) to see if you get success or the same error
• make use of LWP::UserAgent's handlers to better inspect the data. Look at the response_data and response_done handlers -- the wireshark output leads me to believe that some of the data is coming down the pipe (chunked response?) but then the target server throws a 500 so LWP::UserAgent drops the chunks already received (and that my fellow monks is a wild guess!)