faster permutations for Perl 6

permutations.p6

sub permutations(int $n) {
    my int $i;
    $n == 1 ?? ( (0,), ) !!
    gather {
        my @permutations = permutations($n - 1);
        while $i < $n {
            my Int @i;
            my int $j;
            @i.push($j++) while $j < $i;
            $j = $i + 1;
            @i.push($j++) while $j < $n;
            take (nqp::clone($i), |@i[@$_]) for @permutations;
            $i = $i + 1;
        }
    }
}

using @permutations, we avoid computing permutations($n - 1) several times