This is very nice. I thought I'd add a few tips of my own:

• Caching is the enemy of benchmarking. Make sure that the code you're benchmarking doesn't do any caching of the results. This is particularly important if you're using code that other people have written (e.g., modules from CPAN) as part of the code that you're benchmarking.
• Benchmarked code uses package global variables. This is extremely important to note, because if you use lexicals, your benchmark results will mean nothing, because you'll most likely be using undefined values, or values that you're not trying to test. So this goes along with turnstep's recommendation to make sure that your code works before you benchmark it: make sure that it works *while* you're benchmarking it. Most of the time, I use a loop count of 1 the first time I run a benchmark, then I print out the values within the code reference (or string) to make sure I've got everything right.
• Don't intermix eval'd strings with code references, because, according to the Benchmark manpage, code references will show slower execution times than the equivalent eval'd strings.