Anonymous is correct. Perl is a server side app, and via the HTTP protocol there is no way for Perl to be able to check anything on the client's machine. It's even unable to check what browser you are running, the web server tells perl that.
The best that you can do is what btrott said. One way to do it is to set a cookie AND info in the links o the pages, and when a user clicks on a link, you can check to see if there is a cookie or not. If not, keep the user info in the query_string of the links, if so, you can remove the query_string stuff.
Some thoughts to play with....
We put the 'K' in kwality!