Looks good. But it seems that you're polluting the hash with keys KEY, VALUE, CURRENT_STATE and RESCAN_NEEDED. Although the chance that an actual application would use those keys is small, it is not 0.

You might want to bless the object as a list ref with two hash refs: one with the actual hash info, and one with your state maintenance. That way they will never be able to interfere with each other. But this will slow down access to the hash even further though. YMMV.

Liz