You might consider this:

If you build the cobbled-together solution carefully, putting good wrappers around the disparate pieces of the system and writing careful tests for all aspects of the system's functionality, you'll learn more from building the prototype than you would from a month of pure design time.

Also, remember that Perl and C are far from being mutually exclusive. If you can get a running system in Perl, you can run a profiler on your running system and find the parts that need to be optimized. Then, if you need more speed than Perl can provide, you can use XS or Inline to port only the code that you need to C.

I don't mean that good up-front design isn't important; it is. But getting a solution going in Perl, even a slapped-up prototype, can greatly aid a good design, and frequently can be turned into a final solution step-by-step.

stephen