Short of help from the file system or modifying use and require yourself, there isn't any easy way to do this. You could try searching all text files on your system for the module names, but even then you have to account for all the syntaxes (syntaxi?):

use module;
use module VERSION;
use moudle LIST;
require module;
require "path/to/module.pm";
[download]

require with a quoted path leads particularly to madness, when you consider all the paths you'd need to check. And that doesn't account for other ways the code could get loaded via do and so on.

I've just convinced myself that I could modify Perl to do this faster than I could write an exhaustive search :)