%Config::Config like @Config::ISA lives in package Config; while %Config lives in package main;
When you use Config; it exports %Config into your current package (main). If you use package Huang; use Config; it exports (creates) %Config in package Huang; The fully qualified name of this exported (created) %Config is %Huang::Config;
It is a global variable.
This "exporting" is a copy. Copies take memory. So if you use Config(); or require Config; no copy gets made in your current namespace, but you can still access the original by its full name %Config::Config;
Read Tutorials: Variable Scoping in Perl: the basics, Coping with Scoping