they all say you'll be able to run Perl5 scripts by perl6

For that, plain old Perl 5 is used, albeit automatically. This is a feature of the implementation as it is planned, not of the language.

But conceptually I think there was a huge gap between Perl4 and Perl5. You've got real references, real datastructures, real lexical variables, objects

Those were all additions to the existing language. Yes, it was a big step, and there was a huge gap in between. However, the change was almost fully backwards compatible. It's like changing your house by adding a few floors on top of it. Perl 6 on the other hand will redesign the bottom levels as well, because the Perl 1..5 house was built with Jenga blocks. I think the change from 5 to 6 is more than a huge gap; I think a new language is being designed.