In general, use lib is more favorable. (Indeed, in “taint mode” it is required.) The reason is... that “the program always works.” Its success vs. failure isn't governed (nor can it be usurped...) by “something in its surrounding environment.”

Bear in mind, as usual, that in a Windows environment Perl still “speaks Unix,” i.e. with forward slashes, precisely so that source-code doesn't have to be changed when moved from one to the other. Many Perl modules are provided, e.g. in the File:: name-space, which provide platform-agnostic handling of such concerns.