my $string1 = '01010110111000';
my $string2 = '11101101100010';

my ( $c00, $c01, $c10, $c11 ) = (

    ( $string1 | $string2 )  =~ tr[0][0],      # count 00
    ( ~$string1 & $string2 ) =~ tr[\1][\1],    # count 01
    ( $string1 & ~$string2 ) =~ tr[\1][\1],    # count 10
    ( $string1 & $string2 )  =~ tr[1][1],      # count 11

);

##</code><code>##

my $len = length $unpacked_string;
my $packed_string = pack qq{b$len}, $unpacked_string;

##</code><code>##

my $unpacked_string = unpack qq{b$len}, $packed_string;