How to chlorinate a tankless water heater?
Well, the standard method of chlorinating well systems to kill bacteria in the pipes is to use chlorine (peroxide is an oxidizer, but it's not "chlorinating" in my vocabulary) in the well, and pump it throughout the system, wait several hours, and then flush it out.

Presumably you could use peroxide if you have a sufficient amount, but it's likely to be more expensive than using chlorine bleach (plain, unscented as I would hope would be obvious.)

Interestingly, "overkill" generally is less effective at killing things that a proper level of chorine. My particular well with 200 feet of 6" well full of water above the pump takes about 3 quarts of household bleach. Process is to open the well head, run a hose to the well head and turn it on, and add bleach with the hose running so it gets circulated. Once the chlorine is well-mixed in the well, run water from each tap until you smell the chlorine, then shut it off. Wait several hours, pump most of the chlorinated water out with the hose to somewhere that isn't your septic system or a body of water (run it onto the ground), then flush the pipes in the house once there's no more smell of chlorine from the hose.

I'll go look for references and edit.

http://www.dep.pa.gov/About/Documents/Disinfection%20of%20Home%20Wells%20and%20Springs.pdf

http://www.ct.gov/dph/lib/dph/drinking_water/pdf/Well_Disinfection.pdf

This publication suggests a smaller dose and longer contact time:

http://www.oregon.gov/oha/ph/HealthyEnvironments/DrinkingWater/Operations/Treatment/Documents/CT.pdf