If ingested, solutions of hydrogen peroxide up to concentrations of 9% are generally nontoxic; however, even a 3% solution is mildly irritating to mucosal tissue and may cause vomiting and diarrhea. Ingestion of industrial-strength solutions (≥10%) causes systemic toxicity and has been associated with fatalities.
http://www.jtbaker.com/msds/englishhtml/h4070.htm
Large Oral doses can be irritating, and cause vomiting, but if it was a solution of half peroxide and half water, there is another cause for the vomiting.

With that weak of a solution you would have to ingest large amount of the solution. Something your snake would most likely not do.

If you handled your snake after eating, or there are internal parasites, that can be a reason for regurging. I doubt that the bath made them do this as it's barely toxic at all at those levels.