There are two ways to take a reference to a hash.

If you use the \%hash syntax then you are taking a reference to the _original_ hash. Any updates that you make through that reference will obviously update the original hash (as, actually, there is only ever one hash).

If you use the { %hash } syntax then you are creating a new anonymous hash which is a _copy_ of your original hash and getting a reference to this new hash. Any updates you make through this reference will update the copy of the hash and not the original hash.

I recommend reading "perldoc perlreftut" and "perldoc perlref" to clarify these concepts.