Original:

sub gen_idx_powerset (Int $size is copy) returns Array {
    my @c = ([]);
    for 0 .. $size-1 -> $i {
        push @c, (map { [@$_, $i] }, @c);
    }
    return @c;
}
[download]

Converted to gather/take

sub gen_idx_powerset (Int $size is copy) returns Ref {
    my @c = ([]);
    return gather {
        take @c[0];
        for 0 .. $size-1 -> $i {
            take for map { [@$_, $i] }, gathered;
        }
    };
}
[download]