While advice over which topics to cover are appreciated, I am more interested in how to deal with the human element.

Kinda mixing the two together. Before you decide what to teach, canvas a few of the Java guys you get on with and ask them what bits of their day to day workload they most dislike. Chances are that there will be some mundane but essential, currently manual administrative tasks they have to do but find to be a PITA. If you're lucky, one or two of these will lend themselves to being automated using Perl.

If you can start your demo by writing something that has immediate benefit to their daily lot, you'll get an immediately enthusiastic audience. If you demonmstrate creating it in realtime, having carefully worked out your solution previously to ensure your demo goes smoothly, so much the better. The real trick here will be winkling out one or two good candidates.

By starting with something that their main language doesn't really lend itself to, you'll avoid coming across like you are telling them "Perl is better than Java". If you can do that, you may avoid some reluctance/resistance.