in reply to
Safely altering the behaviour of heavily used modules
I think you're going about this the wrong way: this is not a backwards-compatible change, and since Storable is a very heavily used module there are bound to be many STORABLE_thaw methods out there that are not on CPAN, and that will be broken by this change.
The only way you could make a backward compatible change to the way STORABLE_thaw is called would be to change something that would otherwise be guaranteed to be an error, and I think that is likely to be difficult to arrange.
Alternative approaches: add a new method that Storable could look for as well as/in preference to STORABLE_thaw; add a completely new interface to Storable; or subclass Storable.