>perl -wMstrict -le
"use Test::More tests => 2;
 use Data::Dump;
 ;;
 my $string = 'ABCCCDEAB';
 ;;
 my %pairs1;
 $pairs1{$_}++ for $string =~ /(?=(..))/g;
 ;;
 local our %pairs2;
 $string =~ m{ (?= .. (?{ ++$pairs2{${^MATCH}} }) (*FAIL)) }xmsp;
 ;;
 my %pairs3;
 for (my $i = 0;  $i < length($string) - 1;  ++$i) {
   ++$pairs3{ substr $string, $i, 2 }
   }
 ;;
 dd \%pairs1, \%pairs2, \%pairs3;
 is_deeply \%pairs1, \%pairs2, '1 & 2, same results';
 is_deeply \%pairs1, \%pairs3, '1 & 3, same results';
"
1..2
(
  { AB => 2, BC => 1, CC => 2, CD => 1, DE => 1, EA => 1 },
  { AB => 2, BC => 1, CC => 2, CD => 1, DE => 1, EA => 1 },
  { AB => 2, BC => 1, CC => 2, CD => 1, DE => 1, EA => 1 },
)

ok 1 - 1 & 2, same results
ok 2 - 1 & 3, same results