The special treatment will be gone (running once before testing the condition). To avoid that people blindly copy over the Perl 5 idiom to Perl 6 and then complain that it "doesn't work", do-while will be made impossible. Even though I understand that this way less people will complain, I am against such arbitrary limits: with *less* code in the core, *more* can be done.

If anything, I believe it should be a warning in the perl5 category. A category that is on by default at first (even without "use warnings"), loaded with other warnings (only with "use warnings") as Perl 6 becomes more popular, and only loaded when explicitly requested (only with "use warnings :perl5") when we start thinking of Perl 5 as we do of Perl 4 now.

The replacement is obvious and much easier to understand, because it doesn't depend on a special exception made for do-while:

{
    ...
    ...
    redo if EXPR;
}
[download]

I recommend writing it like that even in Perl 5.