First, are you sure your mail client handles RFC 1522-encoded headers correctly?

Does round-trip encoding work as you expect?

print MIME::Words::decode_mimeword(
      MIME::Words::encode_mimeword($self->to, 'Q', 'utf-8'));
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See the warning about encode_mimewords (plural) not being RFC 1522 compliant. Try this with a single word first, and then with several words.