Code writing exercises are all very well, but something that is often neglected are debugging exercises. Start with code that exhibits various bugs and have your team find and fix the bugs.

A valuable lesson along those lines is to present some code without strictures that is to be debugged. Bugs involving incorrect reference usage are particuarly good for this. If they don't figure out to add strictures for themselves, after they have struggled for a while add use strict; use warnings; and show them how to catch many types of error without even running the code.

In the same sort of sense, reinventing wheels can often be good exercise. For example: CSV data is nasty stuff to parse. You could write a number of CSV data sets of varying nastiness and have your team write code to parse the data. Present the data sets in order of nastiness to see if you can break each itteration of the code. At the end of the day present a solution using one of the CSV modules to show why reinventing the wheel is a bad idea!