Some years back I had a Spanish instructor who, earlier in her career, had been in the position of teaching an introductory Spanish class at the high school level while taking an introductory Spanish class herself at night school. The strategy she relied upon was to take what she learned, slow it down, and pass it on. However it was critical in doing so that she had highly organized lesson plans to minimize the possibility that one of the students would take the material off course, straying from her meager knowledge at the time.

Your friend faces a similar challenge, worse actually, since not only is he unknowledgeable about Perl, but his experience is with languages that are very different and much easier. What he'll need to avoid at all costs is some student asking "Can I also do it this way?", because the answer will very likely be "yes" (hey, it's Perl!), but he may not know why or if it's a good idea (yet).

So, my main advice is to work very hard to nail down all the loose minutes in every class period, have a clear set of objectives to achieve every day, and a well thought out plan of how specifically to get there. Granted, this is just good teaching practice, but the advantage that a seasoned instructor (of a particular subject) has over the novice is the ability to allow students to express their creativity and curiousity, and then to lead them back home from tangents. Your friend can not realistically expect to be able to do that.

Also, encourage him to create an account here. I'm sure I'm not the only one who would be willing to hash out lesson plans.

Update: jeffa++ for concrete suggestions on keeping a class on track.