The difference to me would seem to be between "You must go to this on your off time" and "You must go to this on what's normally your off time, but we're going to try and make it up by picking up lunch". It's an acknowledgment that they're asking for extra and compensating for it (maybe not completely but at least making the gesture).

My hangup would be that a lunch hour sounds too short a period of time to cover "enough" (but that'd depend of course on how much you're trying to cover as well as how quickly the learnees are on the uptake). I tried a similar "Here's Perl" talk a couple jobs ago at the company meeting, but because of the time constraint it was a few slides of syntax, a couple on modules like LWP or IO::Socket, and then some pointers to where to learn more. But again, that's dependent on what you're trying to cover and they might let you have several sessions so it could be moot (just tossing it out there though).