Why perlmonks isn't using them?

Because PerlMonks isn't guarding any significant secrets, besides the combination to the St Wall shrine's maintenance access panel.

Seriously, we allow posts by Anonymous Monk, which doesn't even require any kind of registration. The site has a moderation policy that seems to work fine for dealing with the occasional grafitti, and we use simple login techniques that also seem to work fine for our lightweight needs.

The account numbers for the saintly fortunes are all stored off-site. ;)

I'd second Kerberos ... RADIUS and LDAP can handle authentication, but they're not really geared for the same sort of thing (where the authenication service doesn't necessarily trust the server that's doing the authentication).

As for reasons why Perl Monks isn't using it, I couldn't say, but it was discussed a while back in Single Sign-On?

As for my experience w/ remote authentication, it's mostly in LDAP, and I've done a little work w/ RADIUS, but I'm no expert in it. (they're simple, and if you control all of the servers that are doing the authentication, they're fine)