Requiring a login before posting will have very near zero impact on spam at PerlMonks. Sometimes spam is posted anonymously, sometimes not. Our spam tools have been made (by necessity) to deal with both anonymous users and new users.

It gets rather tiring to over and over again read people proposing the obvious solution to the spam problem based on having paid little attention and doing very little thought, as far as I can tell.

Just because 4 of the last 5 spam posts you noticed all had feature $X in common, you cannot conclude that banning $X will cut spam by 80%. It will prevent a couple of spam postings and require a minor adjustment from the spammer, which is likely to happen quickly and thus lead to absolutely no reduction in spam. Frequently, it even increases the amount of spam because the spammer has to pay a little attention and try some changes.

And I think it takes only a little contemplation and having paid a little attention to realize this. In the case of logging in, the adjustment is less trivial... except it has already been made. The transition from anonymous posting to logged-in posting rarely takes a spammer much time these days.