my %charmap = (
    A => '00',
    C => '01',
    G => '10',
    T => '11',
);
sub packACGT {
    ( my $in = shift ) =~ s/(.)/$charmap{$1}/ge;
    print length $in, ':', length( $in ) % 8;
    pack 'Cb*', ( 0, 6, 4, 2 )[ length( $in ) % 8 / 2 ], $in;
}

my %digmap = (
    '00'    => "A",
    '01'    => "C",
    '10'    => "G",
    '11'    => "T",
);

sub unpackACGT {
    my( $len, $out ) = unpack 'Cb*', shift;
    print $len;
    substr( $out, -$len ) = '' if $len;
    $out =~ s/(..)/$digmap{$1}/ge;
    return $out;
}

my $string = $ARGV[0] ||  'GATTACCCC';

print length( $string ), ':', $string;
my $compressed = packACGT( $string );

print length( $compressed ), ':', "'$compressed'";

my $decompressed = unpackACGT( $compressed );

print length( $decompressed ), ':', $decompressed;