If I play Dr. Light and my opponent is Harley Quinn, does her ability trigger?
Dr. Light (paraphrased):
Attack: Choose a foe. That foe puts a location he controls into his discard pile.
Harley Quinn (paraphrased):
When you discard a card during any player's turn, draw a card.
Since Dr. Light doesn't explicitly say "discard", does his attack constitute my opponent discarding to trigger Harley Quinn?
Correct, it does not trigger Harley's effect.