I assume this is directed at the author of rak, so I'll answer.

$x ~~ $y is basically syntactic sugar for $y.ACCEPTS($x). Using ~~ involves an additional subroutine call, so there's a short-term efficiency issue (as in programs running more than a second or so the extra call will have been inlined).

But personally, I like the verbosity of $y.ACCEPTS($x) as it reads better for me, having been burnt by ~~ semantics in Perl, which was symmetrical.