The Perl 6 Synopses 4 contains an example under “The do-once loop”:

@primes = (do (do $_ if .prime) for 1..100);
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Depending on your current level of Perl savvy, this either looks bizarre or deceptively simple. This essay will break it down to its most elementary language features. Like dissecting a frog, there will be a lot of interesting things pulled out and pinned to the mounting board along the way.

You can find the essay on my web page. I'll continue updating it this weekend based on any comments.

—John