Declaring a variable twice in the same scope is indeed an error ...

By who's definition? Certainly not Perl's. Or strict's.

and of exactly the same type as 'strict' is supposed to prevent.

And yet -- it doesn't. Which could mean one of two things:

1. The strict module has had a bug all these years and nobody noticed until you did.
2. You've misunderstood.

I'll let you decide which you want to believe.

And while you do that, I'll continue to believe the evidence of the implementation, and bet you that nothing you say or do will make it change in the future.