I'm not sure I really understand the question. If you're simply talking about accessing a package-global variable  $MyPackage::var from anywhere in the program (which is what being global means), then assign the variable a value with a statement like
    $MyPackage::var = 42;
located anywhere in the program, then, at a later time (i.e., after it's been assigned a meaningful value), access the variable with a statement like
    do_something_with($MyPackage::var);
located anywhere in the program.

I suspect you actually have something a bit more complicated than this in mind.