It's shorthand for "Control-d", or unix End-of-file character. rupesh was using perl as an interactive processor. (Type "perl" at a command prompt, enter the code, then press "Control-d"). I think windows might use CTRL-z or somesuch, but I don't actually.... care, tbh.
davis
Kids, you tried your hardest, and you failed miserably. The lesson is: Never try.