If you can avoid that anyone writes to your hash directly instead of using the accessors of the class then it is relatively easy. You can either use a log or have the hash store not only the value but a serial number/sequence value (concatenated or in a HoH or HoA structure).

If not, you have to use 'tie' to add that functionality, i.e. tie that hash to methods that use an underlying data structure as suggested above, a log or a HashOfHashes with sequence values

Naturally you can't access the sequence numbers from the hash but from additional methods that access the underlying data structure

See 'perldoc -f tie' for more information