Following my curiosity, I rechecked the registration form because I have a mixed memory about using CAPTCHA. And there's no CAPTCHA there. Now I remember that this memory mixing is because at the same time I registered at PM, I registered at other services. Some of them using CAPTCHA.

because the only way people found out about Perlmonks was, literally, by word of mouth

There's a direct link to Perl Monks from http://www.perl.org, right there on the front page. And I believe this is not the only one, now and later.

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Update: added note about link on Perl.org