Apologies - I should have explicitly stated that "working around it in the caller" is not a solution.
It's imperative that, wrt the given pseudo-example, my $n2 = $B_obj - $A_obj; return a module A object with a value of -10.
Hence my agreement with sw1's labelling of both his and my alterations to the caller as being "not practical".
If module B did not overload the '-' operator then, AIUI, my $n2 = $B_obj - $A_obj; would invoke module A's overloading of that operator - which is exactly what I want.
I was hoping there might have been some way of triggering that same behaviour when module B does overload that operator .... but it seems that's not the case.