How did you determine that the white wire is not a neutral? A neutral will trip the breaker when it touches the ground, same as a load wire (which will spark and blast a hole in your pliers if the breaker is on). Both the load and the neutral should be capped if the breaker is on.

You mentioned that it's hooked to a single pole switch. Is it a three way switch, or a single pole? If it's a single pole, that should be neutral. If it's a 3-way switch (where two switches control the same light), that white wire is probably a traveler and your neutral is buried in the switch box. Some electricians will do this because 2-wire is cheaper than 3-wire. If that's the case, you'd have to run a new 3-wire line from the box to the switch, then tie in the neutrals so that you have a neutral, a load and a traveler.