native_utf8_on

Force SVf_UTF8 on in place, without touching the underlying bytes.

Returns the previous flag state as a boolean. Use this only when you know the bytes really are well-formed UTF-8 — flipping the flag on garbage produces mojibake that will surface later as double-encoding or Wide character warnings.

The leading underscore in the name is a warning: this is an internals-poking operation, not part of the everyday API.