To do this kind of context detection, the new want function would need to be able to report that a particular closure was called in the argument list of a particular subroutine.

That is, you'd need to be able to write something like:

    sub findone (&matcher, *@list) {
        ...
        do_caching() if want.arg_of =~ ('while','until');
    }
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I don't see any reason why this kind of (compile-time discernible) information shouldn't be made available, especially since creating new control structures via subroutines will likely be far more common in Perl 6.

I'll see what Larry thinks.