Perl Cookbook, 2nd Edition

Chapter 12. Packages, Libraries, and Modules

Like all those possessing a library, Aurelian was aware that he was guilty of not knowing his in its entirety.

—Jorge Luis Borges, The Theologians

Introduction

Imagine that you have two separate programs, both of which work fine by themselves, and you decide to make a third program that combines the best features from the first two. You copy both programs into a new file or cut and paste selected pieces. You find that the two programs had variables and functions with the same names that should remain separate. For example, both might have an init function or a global $count variable. When merged into one program, these separate parts would interfere with each other.

The solution to this problem is packages. Perl uses packages to partition the global namespace. The package is the basis for both traditional modules and object-oriented classes. Just as directories contain files, packages contain identifiers. Every global identifier (variables, functions, file and directory handles, and formats) has two parts: its package name and the identifier proper. These two pieces are separated from one another with a double colon. For example, the variable $CGI::needs_binmode is a global variable named $needs_binmode, which resides in package CGI.

Where the filesystem uses slashes to separate the directory from the filename, Perl uses a double colon. $Names::startup is the variable named $startup in the package Names, whereas $Dates::startup is the $startup ...