It is a double loop; what's your point? This isn't "come up with the most efficient way to do X", or "do X without using a nested loop", it is (at least so far as I am interested) "show me idiomatic ways to do X". To quote Ovid, "I am wondering how different types of languages would solve the problem of identifying elements in one array that exist in another". I suppose Perl 6 isn't a different type of language than (recent) previous versions of Perl, but I thought the DWIM aspect of the Perl 6 code made the example worth posting.
Sorry if I offended you, but I was under the impression that Ovid wanted to see efficient solutions when he said:
Two 100 element arrays leads to 10,000 iterations. So, how do you solve the scalability issue?