The headers may have been equivalent, but they weren't identical. The missing piece is the Accept header.

rfc2616 says: If no Accept header field is present, then it is assumed that the client accepts all media types.

It will work if you add headers to say that explicitly

$mw->{ua}->default_header('Accept' => "*/*");
[download]

This is clearly a bug with that webserver, it doesn't implement HTTP/1.x as it claims.