See DebugCGI to see what CGI.pm does, to Dumper $cgi , it is trick from Basic debugging checklist )

Make sure the browser is sending correct request with Wireshark · Go deep. or Web Testing with HTTP::Recorder , installing HTTP::Recorder or whatever

Check your perl and config against these checklists CGI Help Guide , Troubleshooting Perl CGI scripts , brian's Guide to Solving Any Perl Problem

Check your html with http://validator.w3.org/

Check your IIS7 settings -- I don't have a checklist for this

Between all these things to check, you ought to figure out if the problem is:
with your html , which causes the browser not to send the correct thing
the browser sending the wrong request despite the correct html
CGI.pm not returning the correct output
The IIS7 server not giving CGI.pm the correct input
An IIS7 server setting you have to enable/disable