Not so much that you missed something but that you didn't know you were missing something. If you look at the docs for Net::SMTP::auth() you'll see it mentions SASL. It doesn't mention needing Authen::SASL installed though, so there's no real reason to think you need that.

However, when you get the "Can't Locate" message it usually means you need to install something (or reinstall it properly). I have Net::SMTP on my machine and it works fine without Authen::SASL installed. But I don't use auth().