my %bitMap  =  ('00' => 'T', '01' => 'C', '10' => 'A', '11' => 'G');
my @byteMap = map{ 
    join '', map $bitMap{ $_ }, unpack '(A2)4', unpack 'B8', chr 
} 0 .. 255;
my @wordMap = map {
    $byteMap[ $_ >> 8 ] . $byteMap[ $_ | 0xff ]
} 0 .. 65535;

...

my $DNA = join '', map{ $wordMap[ $_ ] } unpack 'n*', $raw;