Just to get the party started, and without putting much effort into it, you can change this:

    next if ! grep { $_<val> == 5 } @combo;
[download]

to:

    next if none(@a) == 5;
[download]

Update: another quick note. Your iterator should be able to be nicely implemented in terms of gather/take. Pugs doesn't have it yet, so I haven't had a chance to play with it enough to be very comfortable, but it would be nice if someone could show how to do that. :-)

Another update: here's my crack at gatherifying your combo generator. I dunno if it's right, but it fits my understanding of how the construct is supposed to work:

sub combo (Int $by is copy, @list is copy) returns Ref {
    my @position = 0 .. ($by - 2), $by - 2;
    my @stop     = @list.elems - $by .. @list.end;
    my $done     = undef;

    gather {
        until $done {
            my $cur = @position.end;
            while ++@position[$cur] > @stop[$cur] {
                --$cur;
                @position[$cur]++;
                next if @position[$cur] > @stop[$cur];
                my $new_pos = @position[$cur];
                @position[$cur .. @position.end] = $new_pos .. ($new_p
+os + $by);
                last;
            }
            $done = 1 if @position[0] == @stop[0]; 
            take [ @list[@position] ];
        }
    }
}
[download]

As we can see, this was a very straightforward change. My understanding is that this should act lazilly -- possibly as a coroutine -- if used in a lazy context. Instead of returning lists, I have it take array references, because I'm not sure if taking a list would Do The Right Thing. Again, this is just my understanding of the feature. I could have this all wrong.

Update on another update: added until loop around body of gather. I think it's correct this time. :-)