Urgle. While you pick a fast algorithm, your implementation is not efficient. There's no need to build @selection element by element, while popping off one element of of @population at the time. Just run $n iterations of Fisher-Yates loop, and return the last $n elements of the array. Furthermore,

if ($swapper < $#population) {
    $population[$swapper] = $population[-1];
}
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is less efficient than

$population[$swapper] = $population[-1];
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Sure, the if statement prevents a needless assignment, but the expected number of needless assignments is

Σi=0$n 1/(@population - i)

which, while unbounded, grows very very slowly if the array to sample grows. OTOH, the number of compares done is linear with the size of the sample taken.

Abigail