my %hash = (
  arr1 => [1, 2, 3, 'a'],
  arr2 => [2, 3, 4, 'x'],
  arr3 => [3, 4, 5, 'e'],
  arr4 => [4, 5, 6, 'b']
);

my @sorted_hash_keys = sort { 
  $hash{$a}->[3] cmp $hash{$b}->[3] 
} keys %hash;

foreach my $key (@sorted_hash_keys) {
  print "\$hash{$key} = [@{$hash{$key}}]\n";
}

##</code><code>##

$hash{arr1} = [1 2 3 a]
$hash{arr4} = [4 5 6 b]
$hash{arr3} = [3 4 5 e]
$hash{arr2} = [2 3 4 x]