sub head ($)
{
    my($url) = @_;
    _init_ua() unless $ua;

    my $request = HTTP::Request->new(HEAD => $url);
    my $response = $ua->request($request);

    if ($response->is_success) {
        return $response unless wantarray;
        return (scalar $response->header('Content-Type'),
                scalar $response->header('Content-Length'),
                HTTP::Date::str2time($response->header('Last-Modified')),
                HTTP::Date::str2time($response->header('Expires')),
                scalar $response->header('Server'),
               );
    }
    return;
}