{
   my %h;
   1 if exists $h{key};
   print("exists:  ", (%h ? "auto-vivi" : "empty"), "\n");
}
{
   my %h;
   1 if defined $h{key};
   print("defined: ", (%h ? "auto-vivi" : "empty"), "\n");
}
{
   my %h;
   1 if $h{key};
   print("true:    ", (%h ? "auto-vivi" : "empty"), "\n");
}

__END__
exists:  empty
defined: empty
true:    empty

##</code><code>##

# 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].