I'm well aware of exists, and how it functions, and that it in no way solves the problem presented. Exists tests for the existence of a hash key, so you can tell if it exists but is possibly undefined, but it does not stop autovivification in any way.
Your example doesn't cause autovivification even without exists. Autovivification is the automatic creation of the underlying hashes and arrays in a multiple level data structure when they are used while accessing a nested data-structure.
For example, given an empty hash %hash, $hash{foo} does not cause autovivification, but $hash{foo}{bar} will automatically create an empty hash and assign a reference to it to $hash{foo}.
Your example doesn't cause autovivification even without exists. Autovivification is the automatic creation of the underlying hashes and arrays in a multiple level data structure when they are used while accessing a nested data-structure.
For example, given an empty hash %hash, $hash{foo} does not cause autovivification, but $hash{foo}{bar} will automatically create an empty hash and assign a reference to it to $hash{foo}.