$ perl -Mstrict -Mwarnings -E'package main; say __PACKAGE__'
main
$ perl -Mstrict -Mwarnings -E'package ::main; say __PACKAGE__'
main
$ perl -Mstrict -Mwarnings -E'package ::; say __PACKAGE__'
::
$ perl -Mstrict -Mwarnings -E'package ::::; say __PACKAGE__'
::::

##</code><code>##

{
	package main;
	sub xxx { 0 };
	package ::;
	sub xxx { 1 };
	package ::::;
	sub xxx { 2 };
}

use 5.010;
say ::xxx();      # 0
say ::::xxx();    # 1
say ::::::xxx();  # 2