Based on your responses to the other posts in this thread, let me try to summarize your question.

When someone requests the page, you want to check for presense of cookies and redirect the user to a registration/login page.

The way cookies work is, if the browser has any cookies for that domain then they would be included in the original request. So you do not need to do the return trip thing that you are asking about. Use cgi.pm to check for cookies and issue a redirect if none a found.