If I were to take a training day on $newlanguage, I would want to know the domains it should be used in, what it excels at, what it has a hard time doing, syntax, idioms, libraries and tools at my disposal, and where to go for more help and information. Finding out what I don't know about the language would be useful. It sounds like your audience has coding experience, so you aren't teaching them how to code, just the proper use of this particular tool.

If your participants have experience in other languages, perhaps it would be worth exploring some tasks where the solution in the language of experience and Perl are different - for loops going over an array for example. Usage of (and when not to use) grep, map, and so on I could see being useful as well.