As you probably know, hashes have not any order of keys by definition, so I see the simplest way to keep minimal key value in a special scalar variable.

But if you require 'special' hash, which naturally stores just one key/value pair, i.e. if new key/value pair is written into hash, one of them with greater key is deleted, you can implement your own hash using tie (http://perldoc.perl.org/perltie.html).