here's how to extend .fmt to treat the last value separately

class List is also {
    multi method fmt($format = '%s', $sep = ' ', $lastsep = $sep) {
        my $result = '';
        my $index = 0;
        for self.list -> $elem {
            $result ~= do given $index++ {
                when 0        { }
                when self.end { $lastsep }
                when *        { $sep }
            };
            $result ~= sprintf($format, $elem);
        }
        $result;
    }
}

say <huey dewey louie>.fmt('%s', ', ', ', and ')