Sorry: not only does "is_utf8" miss the mark in this case, but it looks risky based on how it's described in the Encode man page:

    Messing with Perl's internals

    The following API uses parts of Perl's internals in the
    current implementation.  As such, they are efficient but
    may change.

    is_utf8(STRING [, CHECK])
      [INTERNAL] Tests whether the UTF-8 flag is turned on
      in the STRING.  If CHECK is true, also checks the data
      in STRING for being well-formed UTF-8.  Returns true
      if successful, false otherwise.
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Some folks get nervous when they hear about "messing with perl's internals"...