You're referring to it correctly, but use happens at compile time and you're not changing $::var until runtime. You either need to wrap the assignment in your main program in a BEGIN block, or you need to come up with a better method of passing things to your module (consider an import() routine so you could say use MyModule 'value', or implement accessors of some form (MyModule::setVar( "value" ) for example).

Consult perldoc -f use and perldoc perlmod for further enlightenment.