Wide character at /path/to/websocket_server line 680.
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Should I be doing something different to decode what's already UTF-8? Thank you.

Well, according to the documentation, wide characters are exactly the thing you're supposed to be passing to send_utf8 (the function is a one liner that sends Encode::encode('UTF-8', $_[1])), but I don't have enough information about your code to give you advice on what to try next. The error must be somewhere around /path/to/websocket_server line 680.

You can also try to send bytes as-is using $conn->send_binary, but that may need changes on the client side.