The script also works when I comment out the lines use locale; and use open ':std' => ':encoding(UTF-8)'; (which are superfluous at best, IMHO).

«use locale;» is indeed superfluous since he doesn't do any operations that uses locales (cmp, lc, etc). It's not relevant to the OP's question since it doesn't affect encoding.

«use open ':std' => ':encoding(UTF-8)';» is not superfluous. Part of what it does is necessary, and the other part of what it does is wrong. Specifically,

BEGIN {
   # Wrong, and the cause of the OP's problem. See my reply to the OP.
   binmode(STDIN, ':encoding(UTF-8)');

   # Necessary to encode the returned HTML.
   binmode(STDOUT, ':encoding(UTF-8)');

   # Necessary to encode error messages for the log.
   binmode(STDERR, ':encoding(UTF-8)');
}
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It could be replaced with the following or something equivalent, but it shouldn't be eliminated.

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