I've been reading up on this. You definitely should NOT puncture the can. It seems the trick is to be certain the can is submerged at all times, adding water as needed since some water will boil off and the pot shouldn't be covered. Keep the water at a simmer, not a fast rolling boil, and let the can cool down in the water after cooking.
Here's a good recipe with very helpful comments http://allrecipes.com/recipe/caramel-sauce-2/
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