|Keep It Simple, Stupid|
I'm sorry that you have no clue about CPU load. But a redirect is not cost-free. It means that there are two page loads (with an attempt to make the first one have low expense, of course). That's actually a ton more CPU than the tiny addition in the middle of the existing code for rendering user content (and the total CPU for rendering user content is a rather tiny part of the server load for rendering a page).
And that's even ignoring the extra CPU extravagantly wasted by CPU-guzzling bloated American-style browsers and all of the international network infrastructure that needs to be invoked for a second request! And some of that network gear will be located in poor countries that can barely afford such luxuries!
Though I also see value in having perlmonks.org redirect to www.perlmonks.org
Actually, I wasn't proposing only one solution and I actually had no comment at all (yet) on the cookie domain stuff.
As for external links, there isn't a single external link on the internet that will result in me visiting PerlMonks "logged in". That is for security reasons. But... nevermind.
Everybody getting their panties in a wad about CPU usage that will really be completely lost in the noise if you tried to measure it has made this whole thing fall into the category of "unfun" and so I'll go extravagantly waste my CPU somewhere else, now.
In reply to Re^5: Perlmonks 'Not logged in Error' - Proposal for fix (cpu)