Is it possible that perl was having to eval the strings each time the sub was called, thus skewing the results?

No, that's not the way how the Benchmark module works. What the module does is, once it knows how often to run the code fragments (easy if the first argument is positive, and requires some test runs if the argument is negative), it creates, as a string, a loop of the form:

    for (1 .. $n) {$code}
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where $n is the number of times to run, and $code the code you gave as an argument. And then it string evals that. There's no skewing due to re-evalling the string.

OTOH, if you use subs as arguments, you are skewing the results, as the sub calls themselves can easily have a significant effect on the runtime of the code you want to test.