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    $dna = join '', 
           map $conv{$_},
           unpack('(A2)*', unpack("B" . $bytes * BITS_PER_BYTE , $raw)
+);

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

## Omit the braces add a comma for a negligable further improvement
my $DNA = join '', map{ $byteMap[ $_ ] } unpack 'C*', $raw;

my %bitMap  =  ('00' => 'T', '01' => 'C', '10' => 'A', '11' => 'G');
my @byteMap = map{ 
...
...

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