My receiving script works fine so I'm not really debugging 2 scripts. I verified the receiving script using a normal HTML multipart/form-data form to post to it.
The problem lies with the sending script, which I've been fiddling with for awhile now.
I checked the server log: no requests for the 2nd script! Also, no 501 errors. This leads me to conclude that the request isn't even happening. It's dying right after or during the actual LWP/UA request.
I read that the header "Client-Warning" is generated by LWP. If this is true, then perhaps there's really something wrong at the sender side? Perhaps a misconfig of the server?