Indeed, friedo is right.
The formfields are generated by javascript, thus www::mechanize cannot handle them. (as you noted as well)
The only application (i know of) that can be used to test javascript from within a browser is watir, written in Ruby.
But anyone with a bit knowledge of Perl should find out that Ruby isn't that hard, and looks a lot like Perl (flamewar follows :) )
"We all agree on the necessity of compromise. We just can't agree on when it's necessary to compromise." - Larry Wall.