-p: Automatic while-loop with printing

The -p flag is just like the -n flag, except the following statement is executed for each iteration of the loop, right after your program is executed:

     print $_;

In other words, the -p flag allows you to modify input from a file or the keyboard, and have the modified result print to standard output, making it easy to use perl as a stream editor, like the program sed, mentioned in Section 1.1