It's actually the length of the URL that's the limit, and it's unequally enforced.

I think the problem with the POST is that Perl breaks the headers and the content for the POST into 2 different packets. The same submission from IE is a single packet.

This might be a case where you'll need to keep buffering enabled. If you find a $|++; in your code, comment it out.

To get the entire header + form out in one packet, you might need to dive in a level lower than LWP, and deal with the raw socket. Here is some info that'll get you started.