A closely related question is how Perl is going to recognize when it has accidentally been fed Perl 5 code rather than Perl 6 code. It would be rather bad to suddenly give working code a brand new set of semantics. The answer, I believe, is that it has to be impossible by definition to accidentally feed Perl 5 code to Perl 6. That is, Perl 6 must assume it is being fed Perl 5 code until it knows otherwise. And that implies that we must have some declaration that unambiguously declares the code to be Perl 6.
Now, there are right ways to do this, and wrong ways. I was peeved by the approach taken by DEC when they upgraded BASIC/PLUS to handle long variable names. Their solution was to require every program using long variable names to use the command EXTEND at the top. So henceforth and forevermore, every BASIC/PLUS program had EXTEND at the top of it. I don't know whether to call it Bad or Ugly, but it certainly wasn't Good.
A better approach is to modify something that would have to be there anyway. If you go out to CPAN and look at every single module out there, what do you see at the top? Answer: a ``package'' declaration. So we break that.
I hereby declare that a package declaration at the front of a file unambiguously indicates you are parsing Perl 5 code. If you want to write a Perl 6 module or class, it'll start with the keyword module or class. I don't know yet what the exact syntax of a module or a class declaration will be, but one thing I do know is that it'll set the current global namespace much like a package declaration does.
Now with one fell swoop, much of the problem of programming in the large can be dealt with simply by making modules and classes default to strict, with warnings. But note that the default in the main program (and in one liners) is Perl 5, which is non-strict by definition. We still have to figure out how Perl 6 main programs should distinguish themselves from Perl 5 (with a ``use 6.0'' maybe?), and whether Perl 6 main programs should default to strict or not (I think not), but you can already see that a course instructor could threaten to flunk anyone who doesn't put ``module Main'' at the front each program, and never actually tell their pupils that they want that because it turns on strictures and warnings.