Perl in a Nutshell, 2nd Edition

Namespaces and Packages

A namespace does what it says: it stores names (or identifiers), including names of variables, subroutines, filehandles, and formats. Each namespace has its own symbol table, which is basically a hash with a key for each identifier.

The default namespace for programs is main, but you can define other namespaces and variables and use them in your program. Variables in different namespaces can even have the same name, but they are completely distinct from one another.

In Perl, a namespace is held in a package. By convention, package names start with a capital letter, and you should follow that convention when you create your own packages.

Each package starts with a package declaration. The package call takes one argument: the name of the package. Within the scope of a package declaration, all regular identifiers are created within that package (except for my variables).

From inside one package, you can refer to variables from another package by “qualifying” them with the package name. To do this, place the name of the package followed by two colons (::) before the identifier’s name, e.g., $Package::varname.

If the package name is null, the main package is assumed. For example, $var and $::var are the same as $main::var.

Packages may be nested inside other packages. However, the package name must still be fully qualified. For example, if the package Province is declared inside the package Nation, a variable in the Province package is called as $Nation::Province::var ...