When we pass a literal number, for example MODIFY(4), what do you call that?

I call it "argument", just as any non-literal argument. The difference is subtly hinted at in perlsub:

In a subroutine that does not use signatures, any arguments passed in show up in the array @_. Therefore, if you called a function with two arguments, those would be stored in $_[0] and $_[1]. The array @_ is a local array, but its elements are aliases for the actual scalar parameters. In particular, if an element $_[0] is updated, the corresponding argument is updated (or an error occurs if it is not updatable).

Maybe your original question stems from this, as "updatable" is more or less the same as "lvalue"?