# "$ _" is really "$_", and change the qq to a double-quote
  $_ = 
  "CG
  T--A
  A---T
  A----T
   C----G
    T----A
     A---T
      and 
       so 
       on
  ";
  @_{A => C => G => T => } = 0..3;
  s|.*(\w).*(\w).*\n|$_{$-++ / 9 % 2 ? $2:$ 1}|gex;
  s|(.)(.)(.)(.)|chr (64*$1 + 16*$2 + 4*$3 + $4)|gex;
  eval