@array = sort { $a->[2] <=> $b->[2] } @array

But, you say you want to find the $i with the smallest z value. It's best to _not_ sort for that:

my $min_i = 0;
foreach my $i (1..$#array) {
  $min_i = $i if $array[$i]->[2] < $array[$min_i]->[2];
}
[download]

(update: Oops. Forgot the ->[2].)

This should be signifigantly faster, especially when @array gets large...