my @strings =
    qw/XXXXXXXXXABXXXXXXX
       XXXXXXXBAXXXBBXXXX
       BBAABAABBXBX/;

for my $string ( @strings ) {
    my @matches = ( $string =~ /([AB]{2})/g );
    printf( "Found %d matches in %s: %s\n",
             scalar @matches, $string, join( ', ', @matches ));
}