Modules are only read from disc and stored in memeory once, no matter how many times a program or module uses them. This is tracked by the special variable %INC (documented in perlvar).

For example... 

use Foo;
#%INC now contains ( 'Foo.pm' => '/absolute/path/to/Foo.pm' )

use Foo;
#perl "sees" that Foo has been loaded from that entry in %INC and proc
+eeds directly to import()

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Also, see use for more details about how it works, specifically it's relationship to require

In reply to Re: modules using things from other modules by FunkyMonk
in thread modules using things from other modules by Anonymous Monk

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