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When tidy outputs "wide characters", Encode::decode('utf-8', ...) on it fails with: Cannot decode string with wide characters at [...]/Encode.pm line [...] ...which causes the tidy output to be missing. Perhaps replacing Encode::decode with utf8::upgrade would be the right thing to do, at least it appears to work locally for me.
Hopefully fixed in http://www.w3.org/mid/E1MmTus-0002PT-U6%40lionel-hutz.w3.org, see commit log message for details.