And also, in all cases like these, look at what exactly is the actual HTTP exchange ... byte by byte ... that is taking place between these two parties.   You can use Perl logging facilities, or an external tool like WireShark (if not encrypted).   It’s way too easy to think that you know what is being passed, but to be mistaken, and thereby to waste way too much time guessing when you could have looked.   :-/   (Certainly, mea culpa on this one!)   Compare this to external documentation of what the exchange should be.