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BEGIN {
   # Wrong, and the cause of the OP's problem. See my reply to the OP.
...
   # Necessary to encode error messages for the log.
   binmode(STDERR, ':encoding(UTF-8)');
}

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BEGIN {
   binmode(STDIN);                       # Form data
   binmode(STDOUT, ':encoding(UTF-8)');  # HTML
   binmode(STDERR, ':encoding(UTF-8)');  # Error messages
}