Many modules by default export a bunch of functions into the current package (whatever has the use Mymodule; in it). By including an empty list in the use you explicitly forbid anything from Mymodule being exported into the package name space. That doesn't stop you accessing the module's functions however. You just prefix the function name with the module name: Mymodule::wibble () for example to call the wibble function from the Mymodule module.

Why bother? Well, modules tend to be designed in isolation so very often functions that perform similar tasks get the same name in different modules. If all the functions were imported from every module that is used in a package then the chance of collisions between function names becomes very high - using name spaces as above completely avoids the problem.

Another advantage of using explicit name spaces is that then it is clear where the function comes from. I always use CGI with an empty list for example and then either use the OO interface CGI provides or call functions using an explicit CGI:: prefix.