Since all packages are global in scope, nesting of packages is not supported. However, you can have two packages, one called `
A`
and another called `
A::B`
, to give an illusion of nesting. This is a naming convention only and implies no necessary relation between the two packages; however, this convention is typically applied to groups of related packages, and the term "nested packages" does not seem wrong in such cases. For example, you could have a module called Math::Matrix for matrix manipulation and another called Math::Poisson that supports an infrastructure for simulating queuing models. The only relation between the two modules is that they both are mathematical in nature; they don't share any implementation characteristics.

The `
::`
notation is used as before to access variables and subroutines of nested packages:

$p = Math::Poisson::calculate_probability($lambda, $t);
print $Math::Constants::PI;

When you say `
use`
`
File`
, recall that Perl looks for a file called *
File.pm*
. If you say, `
use`
`
Math::Poisson`
, Perl looks for a file called *
Math/Poisson.pm*
(directory *
Math*
, file *
Poisson.pm*
). The double colon gets translated to a filename separator, because the colon has a special significance for DOS filenames. Perl imposes no limits on the level of nesting.