not certain about worms, specifically, but many other critters (some reptiles, amphibs and insects) do a deep freeze and thaw out. The super-cool their cells and don't suffer from the frost as long as the temps stay reasonably steady. Have a buddy with reptiles he used to feed salamanders to, they were kept frozen in the fridge and a few would get up and run away when he thawed them out. Don't have a link for you, but there has been research on this, and it explains how turtles overwinter in shallow nests (our frost line goes rather deep) and how insects overwinter. Worms probably do much the same thing.
Best from Mpls, Lisa