@a = (1,2,3,4);
$a = \@a;
$a =~ s/ARRAY/HASH/;
print $a, "\n";
print $#{keys %{$a}}, "\n";  # print out -1, very nice

$b = {"a" => 1, "b" => 2};
$b =~ s/HASH/ARRAY/;
print $b, "\n";
print $#{$b}; # print out -1, very nice