Well, I've looked at your other posts and the only thing I can suggest at this point is to try to match what internet explorer sends absolutely as closely as possible.

Unfortunately, you're a little limited by LWP in what you can change about your request - it isn't possible to remove the TE header, nor can you easily force LWP to use HTTP/1.0. (I know, your IE isn't doing that, but IE has been known to use HTTP/1.0 when doing SSL through proxy connections)

One thing you can do is add an "Accept" header with a value of "*/*" to your requests, though if that were really a problem I'd expect your GET to be failing as well.

I suppose as one final shot, you could try sending in the request with the login information as a "GET" request. Yes, that's completely different from how IE sends it, but you've tried to be nice and identical, and that's gotten you nowhere.

I've got a small https server here on my local network, and I'm going to see what IE and LWP send differently when presented a form like the one you gave in the top post.