# (1) your working example
$res = $ua->request(HTTP::Request->new(GET => $url),
                      sub {$p->parse($_[0])});
# compared with (2)
$res = $ua->request($req)->as_string,
            sub {$p->parse($_[0])};

##</code><code>##

$res = $ua->request($req, sub {$p->parse($_[0])});