sub _ua_content_cb {
    my ($data, $res, $proto) = @_;
    $res->add_content(\$data);

    # Abort the request if enough content is received to parse for status.
    die if length $res->content > MAX_BYTES;
}

my $res = $self->{ua}->post(
        $self->_url, \%fields, Referer => $self->_referer,
        ':read_size_hint' => MAX_BYTES, ':content_cb' => \&_ua_content_cb,
    );