Please read the Writeup Formatting Tips and updated your post, it needs <code>...</code> tags very badly.

I'll try an explanation:

push (@{$year_index {$year}}, $rlEntry)

%year_index is a hash, and you access the pair with key $year.

If there's already an array reference stored in $year_index{$year}, it pushes the value $rlEntry to this array. If not, it creates a new array, stores $rlEntry there, and stores the new array in $year_index{$year}.

This feature is called autovivification.

See also perlreftut on how the nested data structures and their syntax work.