in Perl, when I do use <module name> <ver>;, the system finds the .pm file for the library somewhere in the @INC path.
use <module name> <ver>;
.pm
@INC
Is there a reliable way to which file was actually loaded?
You want to look in the %INC variable, which records the filename it loaded for the libraries you load with do, require, or use. See perlvar for the details.
%INC
do
require
use