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Factorials can be used to calculate all kinds of useful things. A great example is, how many ways can you order a que. The simple answer is n!, or the factorial of n. So if you have a group of four persons, the number of possible ways to order them from first to last is 4! or 24.

Interaction between the program and the user, or with another program is an importang concept in programming. Whether the interaction is through user input or a config file isn't really important. To be useful for anything but learning, a program would need interaction. Sometimes the user interacts with the program only at startup, other times the user interacts with the program all the way until it exits the program. This article will focus on input at the start of the program.

Most programs contain information that is repeated multiple times. It could be a name, such as Jack, or a number such as 3. It is not unusual to have to change that information across the entire program. In such a situation, variables comes in handy. Variables are just like boxes that contain information. To find out what is stored in the box, you just look inside.

Ever since Brian Kernighan wrote the book "The C Programming Language", the basic program to test a new language has been "Hello, World!". "Hello, World!" is a program that prints "Hello, World!" on the screen. So to start of this Perl tutorial, we will learn how to write a "Hello, World!" program in Perl.