[tpm] Stupid question time... Why does @ISA have to be scoped global?

[Madison Kelly]

As I understand it, when you have a package like:

package Aaa::Bbb;

use Aaa::Bbb::Foo;

our @ISA=("Aaa::Bbb::Foo");

...

Package "Aaa::Bbb" will consult it's @ISA to look for packages to look 
for methods in when it doesn't have the requested method itself. As I 
understand it, this would make me think only that package accesses the 
@ISA array. So why then does 'perlmod' say:

Such a package may also derive some of its methods from another class 
(package) by listing the other package name(s) in its global @ISA array 
(which must be a package global, not a lexical).

I've never liked the answer "Just because", so thanks in advance for 
helping to placate my curiosity. :)

Madi