Programming Perl, 4th Edition

Module Privacy and the Exporter

Perl does not automatically patrol private/public borders within its modules—unlike languages such as C++, Java, and Ada, Perl isn’t obsessed with enforced privacy. A Perl module would prefer that you stay out of its living room because you weren’t invited, not because it has a shotgun.

The module and its user have a contract, part of which is common law and part of which is written. Part of the common law contract is that a module refrain from changing any namespace it wasn’t asked to change. The written contract for the module (that is, the documentation) may make other provisions. But then, having read the contract, you presumably know that when you say use RedefineTheWorld you’re redefining the world, and you’re willing to risk the consequences. The most common way to redefine worlds is to use the Exporter module. As we’ll see later in this chapter, you can even redefine built-ins with this module.

When you use a module, the module typically makes some variables or functions available to your program or, more specifically, to your program’s current package. This act of exporting symbols from the module (and thus importing them into your program) is sometimes called polluting your namespace. Most modules use Exporter to do this; that’s why near the top most modules say something like one of these:

use parent qw(Exporter);

require Exporter;
our @ISA = ("Exporter");

These two lines make the module inherit from the Exporter class. Inheritance is described ...