It might be fun to give them a bit of data and then get them to 'roundtrip' it between lots of different formats.
eg, give them a list of addresses in CSV, ask them to populate a DBI table with it, pull the data from the database into a YAML file and then from there to an XML file. Finally produce a CSV file and compare it to the original.
Doing this they will need to use several CPAN modules which all have different calling conventions, YAML exports functions, CSV_XS is OO, XML::Simple allows for chaining. It may not be relevant to your existing code base but it might open up good discussions on the way they did it - which will allow you to gauge how the might work in a team.