use Benchmark qw(cmpthese);

my $str = "wwibblewibblewibblewibbleibblewibblewibblewibble";

cmpthese -1, {
    regex => sub {
        my %count;
        ++$count{$_} for $str =~ /(?=(..))/g;
    },
    substr => sub {
        my %count;
        ++$count{substr $str, $_, 2} for (0 .. length($str) - 2);
    },
    unpack => sub {
        my %count;
        my $n = length($str) - 1;
        ++$count{$_} for unpack qq{(a2 X)$n}, $str;
    },
};

          Rate  regex unpack substr
regex  15316/s     --   -43%   -74%
unpack 26935/s    76%     --   -54%
substr 58514/s   282%   117%     --