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{
   my %h;
...
exists:  empty
defined: empty
true:    empty

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

...
$h{$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].