I'm not certain why you need this, but here's the deal. The $/ is a global variable. Adding the local before setting $/ to "Foo" makes it local to that block and anything you call from that block. The only way I know of to make a global "package specific" is to locally reset it to your desired package version at the start of every method (again using the local keyword.

Personally, I tend to just avoid the globals whenever I can.