Greetings All,
A very interesting and ambitious task to say the least! Though not impossible. I would first research a common problem or task that your Humanities colleagues encounter on a regular basis (you mentioned searching texts). Then write up an example situation and code a script to deal with it, focus on an easy UI and perhaps give it a web interface. This way you have a palatable example (that is easy to use) to show while you proselytize Perl.
If you somehow keep their attention then I would focus on logical comparisons and loop constructs to illustrate that what we do as programmers is not magic, just meticulous... help demystify the syntax (which I find most students get caught up on). This is assuming you are trying to get them to program as well, if not field questions.
Seeing as these are Humanities people I’m thinking you would be better off writing some software for them to deal with their problems rather than attempting to teach them how to program. Just keep your audience in mind. I tutored children with Autism for a while as well as taught an experiment based after school seminar in science for 2-5th graders and keeping in mind the fact that your audience is not as educated, in whatever field you are teaching, as you are is important (this mostly applies to field specific vocabulary, but concepts as well). I would try your presentation out on someone you know that is not tech savvy first.
Hope that helps... a bit.