Note that if you are logged into perlmonks.org and a link takes you to www.perlmonks.org it will work just fine. It's only the other direction that breaks
What you're seeing is the domain value of the cookie (see CGI::Cookie). By default, the domain is the machine you're on. You can explicitly set if anywhere up your food chain. So, as an example - www.perlmonks.org can set it's domain value to perlmonks.org but perlmonks.org cannot set it's domain value to www.perlmonks.org. If you log into perlmonks.org, www.perlmonks.org can see the cookie.
Oren