2.8 Scope of a Package Directive

All files start as if you had said package main;. Any package directive remains in effect until the next package directive, unless that package directive is inside a curly-braced scope. In that case, the prior package is remembered and restored when the scope ends. Here's an example:

package Navigation;

{  # start scope block
  package main;  # now in package main

  sub turn_towards_heading {  # main::turn_towards_heading
    .. code here ..

}  # end scope block

# back to package Navigation

sub turn_towards_port { # Navigation::turn_towards_port
  .. code here ..

The current package is lexically scoped, similar to the scope of my variables, narrowed to the innermost-enclosing brace pair or file in which the package is introduced.

Most libraries have only one package declaration at the top of the file. Most programs leave the package at the default main package. However it's nice to know that you can temporarily have a different current package.[10]

[10] Some names are always in package main regardless of the current package: ARGV, ARGVOUT, ENV, INC, SIG, STDERR, STDIN, and STDOUT. You can always refer to @INC and be assured of getting @main::INC. The punctuation mark variables such as $_, $2, and $! are either all lexicals or forced into package main, so when you write $. you never get $Navigation::. by mistake.