Turned out that HTTP::Tiny does not support Unicode, just raw text.

HTTP has no concept of encoding. It's just a file transfer protocol.

By definition, text files don't have an encoding defined within, so HTTP headers can be used to communicate the encoding of the text file. But that doesn't mean that the HTTP agent should automatically decode the file. And it that doesn't apply to binary files such as XHTML. Even modern HTML is really a binary file.