Not an answer, just a guess and a method for finding out more:

Maybe it's the browser's fault. I'm wondering whether the browser displays content differently depending on whether its HTTP method is GET or POST.

You might be able to test whether your script is doing what you expect even under POST conditions by writing up a mini-"browser" script via LWP::Simple or maybe one of the IO:: or HTTP:: modules.

Hope this helps,

BCE
--Your punctuation skills are insufficient!