Hope I understood the problem, but your code really need the <code>...</code> tags!

You don't understand this: push (@{$year_index {$year}}, $rlEntry); so let's analyze it:

In your code is defined %year_index, the prefix % says it's a hash, $year is a scalar; so $year_index{$year} is the element of the hash with key==$year.

This element is a reference to an array, @{...} dereferences it so @{$year_index{$year}} is an array.

In the end the code means: push the scalar $rlEntry into the array referenced by the element of the hash %year_index with key $year.