This is an interesting idea for a couple of reasons:
While I'm kidding about golf, I don't think that this is a terribly inappropriate thing to say. Golf is kind of like playing speed chess: it can lead to bad habits. Admittedly, golf can teach one about interesting features of Perl that otherwise may remain unknown, but often those unknown features are unknown for a darned good reason.
Dominus has created the Quiz of the Week which pretty much covers what I think you're trying to do, but I see a couple of pitfalls that he's avoided through a mailing list that we might have trouble avoiding.
One pitfall would be controlling the quality of the questions. Who would post the challenges? Someone might post a challenge of "sorting a file by date", but not clarify exactly what they are referring to. What's the file format? How do we handle bad data? What are the file size limits? What's to be done with the output? Coming up with good specifications is hard, so this could be a problem.
The other problem is that "first post" often leads to "worst post". On the quiz I tried with the Quiz of the Week (finding anagrams), as an experiment I deliberately coded my first try with a "first post" mentality and then sat down and planned out how I would really write the program. My "first post" attempt was so ridiculously slow as to be useless. My "planned out" program was more efficient and, if I saw all of the responses, was the second fastest program in the "beginner" category.
Taking the above into consideration, who sets up the challenges and who judges? How do you prevent the "first post" problem? Perhaps more importantly, who's going to add the new section? None of this is insurmountable, but plan it out. Specifications are hard :)