use Benchmark qw(cmpthese);

my $str = 'wwibblewibblewibblewibbleibblewibblewibblewibble';
cmpthese -1, {
    regex => sub { () = $str =~ /(?=(..))/g },
    substr => sub { () = map { substr $str, $_, 2 } (0 .. length($str) - 2) },
};

          Rate  regex substr
regex  13917/s     --   -30%
substr 19910/s    43%     --