Very briefly, methods are functions used in object oriented code (there's OO tutorials in perldoc is you're not familiar with this). Access to methods should be through inheritance - exporting method names will likely break inheritance.

If you read a bit further down that section you'll see it makes a distinction between modules that are object oriented and those that are just a collection of functions. As an example of the latter type, Scalar::Util is a collection of functions which you can selectively choose to export.