use Benchmark qw|:all|;

my @a = (1..1_000_000);
my $b;

my $c = 'x' x 1_000_000;
my $d;

cmpthese( 5, {
              ary => sub {
                 for my $i (0..1_000_000-501) {
                    $b = $a[$i] - $a[$i+500];
                 }
              },
              substr => sub {
                 for my $i (0..1_000_000-501) {
                    $d = substr($c,$i,1) . substr($c,$i+500,1);
                 }
              },
              unpack => sub { my @b = unpack 'c*', $c },
              split => sub { my @b = split //, $c }
             }
        );