I realize you said that you can't get someone to install a lot of modules, but
libwww-perl is completely self-contained, as nearly as I can tell. And within said bundle would be the perfect module you're looking for,
HTML::Form. This will allow you to submit forms, takes care of all the nastiness of making sure the headers are right, was written by Gisle Aas, and is well tested. There's even an example of using it here on the site,
Homenode Updater (shameless self plug).
When I was playing with it earlier, I was using it to submit files, also. If you have any possibility of getting someone to install a module, this would be the way to go.
--Chris
e-mail jcwren