Because Perl methods are just regular subroutines, it's difficult to
tell what's a method and what's just an imported function. As a result,
imported functions can be called as methods on your objects. This pragma
will delete imported functions from your class's symbol table, thereby
ensuring that your interface is as you specified it. However, code
inside your module will still be able to use the imported functions
without any problems.

WWW: http://search.cpan.org/dist/namespace-sweep/