You can undef %hash which will reduce it's reference count by one

If "it" refers to the hash %hash, you're wrong.

• %hash = (); empties the buckets, reducing the reference counts of all elements by one.

• undef %hash; empties and frees the buckets, reducing the reference counts of all elements by one.

However, neither lowers the reference count of %hash. (They could lower the reference count indirectly if you had something like $hash{ele} = \%hash;, but that's not what you said.)

The later is usually overkill.