# If $h{$i} and $a[$i] are not defined,

$h{$i}{$j}  # Creates a  hash  and stores a ref to it in $h{$i}.
$h{$i}[$j]  # Creates an array and stores a ref to it in $h{$i}.
$a[$i]{$j}  # Creates a  hash  and stores a ref to it in $a[$i].
$a[$i][$j]  # Creates an array and stores a ref to it in $a[$i].