my %entry = ($item_id=>\@data, $item_id=>\@data);
print $entry{$i}->[$j]; # contents of the hash ({}) are array references; we use -> to work with references; then we use [] to work with arrays
# OR:
my $reference = {$item_id=>\@data, $item_id=>\@data};
print $entry->{$i}->[$j];
# firstly, $entry is a reference to a hash, use ->
# secondly, hash contents are accessed using {}
# thirdly, it's again a reference
# fourthly, arrays are accessed using []