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use Benchmark qw|:all|;

...
              split => sub { my @b = split //, $c }
             }
        );

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)
...
substr   2.33   117%     --   -26%   -47%
ary      1.72   193%    35%     --   -28%
unpack   1.23   309%    89%    40%     --

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