Since JSON is (supposed to be) UTF-8, you merely need to mark the resulting data as being UTF-8 decoded. You could even do it for all string data, assuming that all your input has been verified as UTF-8. See for example Re: Bypass utf-8 encoding/decoding?, the function/macro you want is newSVpvn_utf8.

Obviously, this implies that you're trusting your input data to actually be valid UTF-8...