That's an interesting question. Here's what I would do, but it may not be the most elegant solution.
I would load that page with
Firefox, select the options that you want on the pulldowns, then select Page Info from the Tools menu. Click on the Forms tab and it should show you the names and current values of all form inputs. I would then build my
WWW::Mechanize request using this information.
It's a bit roundabout and not Perlish in any way, but it should do the trick.
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It's all fine and dandy until someone has to look at the code.