This is happening because you're not dereferencing an array.

Maybe something like:

use strict;
use warnings;
use Data::Dumper;

# This will give you a clearer idea of what you data actually looks li
+ke
print Dumper($AoH); 

# This assumes that $data is an index (integer)
my $year = $AoH->[$data]{'YEAR'};
[download]

If this doesn't work for you, post the contents of the Data::Dumper output, and we'll talk some more.

The value in the incredibly poorly named $AoH{ data } (if anything by the actual type and contents it's an HoA, but I digress) is an array reference not a hash reference, yet you're trying to treat it as a hash ref. You need to take three steps back and rethink your data structure and how you're going to lay things out (Data::Dumper and/or YAML::Syck can be illuminating figuring out what you've really got).

If $AoH is an array of hashes, then your line should be my $year = $AoH[ data ]->{ 'YEAR' } || '';

As it is now, it tries to coerce the array $AoH into a hash, and fails.

Um, wouldn't there be a problem using a bare word ("data") as an array index?

With use strict;, such a thing would not be allowed; without the stricture, the bare word evaluates to zero when interpreted in a numeric context, and this is probably not the real intention...