use strict;
my %charmap = (
    A => '00',
    C => '01',
    G => '10',
    T => '11',
);

my $string = 'GATTACA';
$string =~ s/(.)/$charmap{$1}/ge;

print "$string\n";

my $compressed = pack 'b*', $string;

print "$compressed\n";
printf "%d bytes\n", length $compressed;

# now use vec() to get at the single parts of $compressed

my $decompressed = unpack 'b*', $compressed;

print "$decompressed\n";