I have multiple versions of Perl installed on my machine; rather than installing them on the default path, I usually qualify based on version (e.g. C:\Perl-5.16). That allows for the most fluid transition from one version to another, as well as letting me easily check what's installed.
If PPM didn't put your libraries in the site/lib folder, might I suggest doing a file search for SMTP_auth.pm (pretty unique name) to figure out where it did install them?
#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.