That's two questions... :-)

I would not call that a private subroutine declaration, but an anonymous subroutine stored in a private variable. You can already do the same thing in Perl 5, except you have to say "sub" explicitly. But if you want a private sub, either of

    my &foo = {...};
    my sub foo {...}
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will work.

As for your second question, in general we'll try not to hang specialized methods off of generic classes unless they really belong. Most of these things are multi-dispatched global subs instead. (And while is probably hard-wired in the interests of giving the optimizer as much information as possible, so that it can cheat on control exceptions like "next".)