I agree that it's easier to create the symlink. Botom line:

If the modules / libraries are being used by one user, the environment solution seems best to me.

If the modules / libraries are to be used by everyone on the system, it would be good to create a symlink from your site_perl directory.

If the modules / libraries are to be used by everyone, and you're worrying about security, it would be good to move your modules / libraries to the site_perl directory.