$ENV{PATH} is tainted because it is obtained from "outside" your program. In taint mode, you can't use something from outside your program to affect something else outside your program. Your '.' is explicitly defined within your program, it is not obtained from an outside source (such as a file, an environment variable, or STDIN). Also, use lib is not attempting to affect something outside of the program, it is changing where the program looks for modules. Because of both, this is taint-free (I'm sure about the first part, and pretty sure about the second).

Note that there are a few kinds of data obtained from outside the program that are not tainted. See perlsec for details.