In my code hash variable key is an array reference

No it isn't! It is a stringified version of the reference. In fact in Perl (5) a hash key can only be a string. In the conversion you loose "some" (read: "quite a lot of"!) information, hence the problem you found. The only workaround is to keep track of it yourself, which possibly amounts to useless duplication, or else completely change the logic of your code, which seems more reasonable.