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A.19 Modules and Packages

Chapter 1 summarizes the basic concepts of
namespaces, packages, and modules.

A namespace is a table containing the names and values of variables
and subroutines. Namespaces are excellent ways to protect one part of
your code from unintentionally using the same variable or subroutine
name as appears in another part of your code, causing a namespace
collision and often leading to incorrect (but hard to identify)
program behavior.

By default, a Perl program uses the namespace called
main.

The package declaration enables you to declare and
use different namespaces for different parts of your program. For
instance, to declare a new namespace called Outer,
use the following statement:

package Outer;

Package declarations usually occur at or near the top of a file and
are in effect throughout the file, but they can appear several times
within a file, causing the active namespace to switch each time they
are called.

When a file has one package declaration at the top of the file and
it's named with the package name followed by the
.pm suffix (e.g, Outer.pm), the
file is called a Perl module. (The module also needs to end with the
statement "1;" to load correctly when called.)

The code for a Perl module can be used in a Perl program by
referencing the file defining the module with a
use statement, as in the following example:

use Outer;

The Perl interpreter will then try to find a file called
Outer.pm.

Chapter 1 gives the basic details on how to manage
modules so that the Perl interpreter can find them.