#!/usr/bin/perl
use warnings;
use strict;

use Data::Dumper;

my @array = qw(AB AB CD AB AB AB);

my @result = [ shift @array ];
for (@array) {
    if ($_ eq $result[-1][0]) {
        push @{ $result[-1] }, $_;
    } else {
        push @result, [ $_ ];
    }
}

print Dumper \@result;