In Re: Why cookies do not work on some users' windows PC?, the OP had some kind of compatability issue trying to manually create a cookie header, which he may or may not have eventually resolved.

My guess is that there's some bug in your cookie code, and Mozilla is willing to ignore it while MSIE isn't. You can read through the HTTP cookie specifications carefully, look for some example code, compare your code to working code, etc. Or you can use a module like CGI::Cookie, where the author has already done all of this work for you.