Hum... hard question Lady of the Lake! Earthworms feel pain as a local negative stimuli to retreat from a danger; not as a suffering like humans do. That is because earthworm brains don't have the higher development required for processing painful stimuli.
Lucky for me, my frogs take them whole. I do have decapitated pinced worms in the past and never liked it. Recommend an extremely sharp blade and swiftness to get it over as promptly as possible.