foreach $letter ( split //, shift @ARGV ) {

    @digits = ();
    $num = ord $letter;

    while ( $num ) {
        unshift @digits, $num % 4;
        $num = int $num / 4;
    }

    unshift @digits, 0 while @digits < 4;
    $base4 = join '', @digits;
    $base4 =~ tr/0123/ATCG/;
    $string .= $base4;

}

print $string;