Perpetual means they can indefinitely deliver as many designs as they want using the ISA they licensed. Not "perpetual for everything ARM present and future". It's similar to the perpetual multi-use license except the holder has more freedom with the modifications and customized designs. All other licenses are time limited.

And again, the terms of the license may vary. I have the impression that Apple has a far more permissive license than anyone else out there for example.