They are the same variable. See Config. It exports %Config::Config into your namespace.

%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

This "exporting" is a copy. Copies take memory.

Actually, it only copies a reference, not a lot of memory

perl -MConfig -le " print \%Config::Config; print \%Config; "
HASH(0x99abe4)
HASH(0x99abe4)
[download]

I see. When 'use Config' first, and then in 'package Huang', it must be identified with original path as "$Config::Config". If use '$Config{osname}', it will warn 'Global symbol "%Config" requires explicit package'.

Yup, it all depends what package you're in

$ perl -l
use strict; use warnings;
use Config; # main
package Huang;
use Config;
package Batman;
use Config;
package main;
print \%Config; 
print \%Huang::Config;
print \%Batman::Config;
print \%main::Config;
print \%Config::Config;
__END__
HASH(0x99abcc)
HASH(0x99abcc)
HASH(0x99abcc)
HASH(0x99abcc)
HASH(0x99abcc)
[download]