my %fred =
  (
    [1, 2, 3] = [1, 1, 0],
    [3, 4, 2] = [1, 0, 1],
    [2, 2, 1] = [1, 1, 0],
  );

  my $i = 0;

  foreach my $ref (keys %fred)
  (
    $ref->[2] = ++$i;
  )