Perl 5 also suffers from the "reference implementation as specification" problem, which is why Perl 6 is actually being quite careful to avoid designating a "reference implementation".
I think our (Perl 6's) notion of "standard grammar as specification" is more of a temporary designation than a permanent one. For now we need a rough guideline to follow when the prose description isn't matching the implementation, both of which are still undergoing a fair bit of revision and updating as we work through the implementation details.
The ultimate arbiter in the Perl 6 specification is the official Perl 6 test suite (http://perl6.org/specification/), which is to be shared among all implementations. Basically, if something is to be considered "official Perl 6", it has to appear in the test suite, and any implementation that then passes the test suite is considered a valid Perl 6 implementation.
Pm
(Yes, the test suite is also evolving. But the test suite certainly gives us something that can be reasoned about without having to also adopt all of the quirks of a specific reference implementation.)
