I'm almost positive that this perk is not functioning properly. When I first obtained it, my bow recharged 5% each time I killed an eligible enemy, as it's supposed to. Then, after a couple of hours, with the very same weapon, it stopped recharging completely. I have killed hundreds of enemies now, with my bow's fire/frost damage not activating a single time. Large soul creatures, small soul creatures--it makes no difference. The bow is stuck at 0 charges.

Maybe it can't recharge at 0? Try recharging it with a Petty Soul Gem, then kill stuff.

I can't rule that out. But even if that's the case, I don't think that's how the perk is intended to function.