The login or session info is kept in cookie. Cookie are domain specific (with some exceptions relating to subdomains). If you're visiting perlmonks.com, the server can't set a cookie for perlmonks.org. Your browser simply does not send PM your cookie if you're using the "wrong" domain, so PM has no idea who you are. Some sites "solve" this by redirecting all domains to the "canon" one, so no matter what domain is used to reach the site, all users end up using the same domain. PM lets you pick the domain you'd rather see in the address bar.