Whether or not it's a bad idea on the part of the boss (the OP seems to be a victim in this), it's got to be done. In the best case the person (or people) who wrote the various Perl programs is|are available, have an accurate idea of what they did, and are willing to talk. In the worst case, the original programmer has had an unfortunate incident involving a train. Somewhere in between is the realm where various forms of persuasion may be useful (sweet talk, beer, food, origami swans, pictures of the original programmer in compromising positions, thumbscrews)