First of all, you need to understand packages and modules; see Chapter 10, and Chapter 11. You also need to know about references and data structures; see Chapter 8 and Chapter 9. It's also helpful to understand a little about object-oriented programming (OOP), so in the next section we'll give you a little course on OOL (object-oriented lingo).
An object is a data structure with a collection of behaviors. We generally speak of the behaviors as acted out by the object directly, sometimes to the point of anthropomorphizing the object. For example, we might say that a rectangle "knows" how to display itself on the screen, or that it "knows" how to compute its own area.
Every object gets its behaviors by virtue of being an instance of a class. The class defines methods: behaviors that apply to the class and its instances. When the distinction matters, we refer to methods that apply only to a particular object as instance methods and those that apply to the entire class as class methods. But this is only a convention--to Perl, a method is just a method, distinguished only by the type of its first argument.
You can think of an instance method as some action performed by a particular object, such as printing itself out, copying itself, or altering one or more of its properties ("set this sword's name to Anduril"). Class methods might perform operations on many objects collectively ("display all swords") or provide other operations ...