The request() method of LWP::UserAgent returns an HTTP::Response object. This means you will need to deal with $res as an HTTP::Response object, not a simple string. From the HTTP::Response manpage, the appropriate way to deal with these would be the content() method.

if ($res->is_success) {
    print $res->content;
} else {
    warn $res->status_line, "\n";
}
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See HTTP::Request and HTTP::Response.