sub sms_WxH_PDL_naive ( $m, $w, $h ) {
    my $ret = 0;
    
    for my $r ( 0 .. $h - 1 ) {
        my $r_sel = "$r:" . ( $r - $h );
        for my $c ( 0 .. $w - 1 ) {
            $ret += $m-> slice( "$c:" . ( $c - $w ), $r_sel )
        }
    }
    return $ret
}