This is working very well under OS X 
using Safari and Camino and also under 
XP Pro on IE 6 and NN 4.7 and NN 7.02.

The very fact that your code is working fine in other browser/os combos shows that your code is fine. I have been using CGI::Session for a while now and it is a fine module (I hacked a SQLite driver for it for my use).

As others suggest, this is a user-specific problem. Start there. See if that user is able to use the app correctly using Netscape/Mozilla/Opera, etc. If that works then the problem is narrowed to IE.

Good luck.