A variable is a “word” in a command that begins with a dollar sign ($) and a letter (such as $a), or a dollar sign and an underscore (such as $_). Variable names can be up to 100 characters long, not including the dollar sign ($). Examples of variable names are $docname and $_currentline.

Although a variable must start with a letter or underscore (_), the remainder of the name can consist of letters, digits, and underscores. Uppercase and lowercase letters are distinct; that is, $ABC and $abc are different variables.