I was in the same lecture, it definitely wasn't mentioned. I'd been hacking on Perl for years before starting school, so I already "knew it in my heart" how it worked in practice. Just one of those little but critical details where something important got missed.

And yes, when you've got a complete understanding, it seems quite obvious that the key has to be stored with it. That's why I got it. He'd just learned it though, and there were still those gaps in his knowledge that he would have carried for a long time until someone else helped point out the missing piece (or he had decided to sit through and make sure he understood everything)