It looks like you'll have to modify CBC.pm. In my version (2.14), the 'finish' method deletes the current IV.
in reply to Re^2: Crypt::CBC and varying the key
in thread Crypt::CBC and varying the key
You'll have to prevent that from happening, and then add a method to return the current (last) IV, instead of the initial IV
I have no idea if or how this affects the security of the two messages.
Hope this helps, J