Says merlyn, quoting Lincoln Stein:

Under no circumstances will the contents of the URL query string interfere with similarly-named CGI parameters in POSTed forms.

That's a nice theory, but if the browser screws it up, there is nothing that Lincoln or CGI.pm can do to unscrew it. And in my experience, this is one of the cases that browsers are most likely to screw up.

Many browsers like to take the URL parameters and the posted form data and mingle them in various nasty ways.