Variable as a reference or value to a function

[mdijkens]

The reason your initial code passes by value to the sub is the use of double brackets
Call MySub(a) ' passes reference to a
Call MySub((a)) ' evaluates (a) to tempvalue and passes reference to tempvalue

(Same is true for Functions btw)

btw i prefer to write (and read):
MySub a 'byref
MySub (a) 'byval