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 }
             }
        );

##</code><code>##

Benchmark: timing 5 iterations of ary, split, substr, unpack...
       ary:  8 wallclock secs ( 8.62 usr +  0.00 sys =  8.62 CPU) @  0.58/s (n=5)
     split: 26 wallclock secs (24.93 usr +  0.33 sys = 25.26 CPU) @  0.20/s (n=5)
    substr: 11 wallclock secs (11.64 usr +  0.00 sys = 11.64 CPU) @  0.43/s (n=5)
    unpack:  7 wallclock secs ( 6.14 usr +  0.03 sys =  6.17 CPU) @  0.81/s (n=5)
       s/iter  split substr    ary unpack
split    5.05     --   -54%   -66%   -76%
substr   2.33   117%     --   -26%   -47%
ary      1.72   193%    35%     --   -28%
unpack   1.23   309%    89%    40%     --

##</code><code>##

while ($dna =~ /(?=A.{500}T)./g) {
     print "Got an AT match at position ", pos $dna, "\n";
}