It still exists in the latest version. The docs say

(W) The first chunk parsed appears to contain undecoded UTF-8 and one
or more argspecs that decode entities are used for the callback
handlers.

The result of decoding will be a mix of encoded and decoded characters
for any entities that expand to characters with code above 127.  This
is not a good thing.

The solution is to use the Encode::encode_utf8() on the data before
feeding it to the $p->parse().  For $p->parse_file() pass a file that
has been opened in ":utf8" mode.

The parser can process raw undecoded UTF-8 sanely if the C<utf8_mode>
is enabled or if the "attr", "@attr" or "dtext" argspecs is avoided.
[download]

It could be that that server didn't specify the character encoding of the content.