you should also decide whether you want this 6 months contract to be extended, so that you can calculate the "adjustment" and "learning" time.

I will second tomasz's suggestion that unix-skills are important. Not only because you indicated before that you have such environment in your host but also because automating and standardising procedures (e.g. procedure to export all files from development to production machines, backup or reinstate DB contents, etc.) is important (to me). Some knowledge of security is also important both for backend and frontend.

Unlike others, I would not consider knowledge of C important or desirable for such a small time-span and for the frontend. Neither I would deem R experience important at all. R is a script-like language but that's it. OTOH both C and R would have taught the candidate how to break things up in small functions or package things in modules. And about data structures which is another thing that I saw your code be in need for, e.g. function return data.

On Python: I would be really worried if a candidate managed to persuade me to convert to using it! That said, similarities of Perl and Python could be the main data structures (hash=dictionary, arrays), knowledge of references (that's where C comes handy, re:pointers but Python references I think are transparent, Perl's aren't) how to combine these into complex data structures, functions receiving and returning complex data structures, extending function behaviour easily as the spec changes (e.g. input parameters), modularising into packages, reading config parameters, testing-testing-testing and, of course, OO. All are really important and are crossing the boundaries of programming laguages.

I would consider also spending some pub-beer-time together as part of the interview. The ability to learn new things and improve depends highly on personality. The ability and desire to automate too. We are talking about comp-science projects after all.

And I am going to cramp here my usual OS-hatred tirade: I would never hire anyone using a specific OS for their home desktop on their free time (and, no, Android does not count). I would demand 24/7 exclusive use of specific OS only ... Yes I am an OSist!

Finally, for such a small timeframe, perhaps giving them a concrete task to solve at home, prior to the interview would be a good indicator. For example: "use Template in order to solve some real problem" you faced, or "use this DB schema to setup database, insert mock data and retrieve it (within Perl)". I don't know how would this work wrt paying them for it as it can take a couple of days of work.

discalimer: I have never interviewed anyone, but had lots of others interviewing me.

bw, bliako