Here is the source where I got the code: http://books.google.de/books?id=1GkMNv7PM68C&pg=PT512&lpg=PT512&dq=david+powell+perl&source=bl&ots=xAklm_Ubqq&sig=R1NGO4xKwI8nha8Exm0gOhaVny0&hl=en&sa=X&ei=D9J0U-TUHcuf7AaAvYDAAg&ved=0CDQQ6AEwBw