#!/usr/bin/perl
use warnings;
use strict;
use experimental qw( signatures );

{   package My::A;
    use overload '-' => \−

    sub minus($x, $y, $swap) {
        return ref($x)->new(
            ($y->isa('My::B') ? $x->[0] - $y->{value}
                              : ($x->[0] - $y->[0])
            ) * (1, -1)[$swap]
        )
    }

    sub new($class, $value) {
        bless [$value], $class
    }

    # We can change My::B outside of B, in fact.
    eval q{ package My::B;
            my $m = My::B->can('minus') or die 'My::B not loaded';
            use overload "-" => sub ($x, $y, $s) {
                return $y->isa('My::A') ? $y->minus($x, 1) : $m->($x, $y, $s)
            }
    };
}

{   package My::B; # Can't be changed!
    use overload '-' => \−

    sub minus($x, $y, $swap) {
        my $subtr = $x->{value} - $y->{value};
        return ref($x)->new($swap ? -$subtr : $subtr)
    }

    sub new($class, $value) {
        bless {value => $value}, $class
    }
}

my $a0 = 'My::A'->new(16);
my $a1 = 'My::A'->new(6);
my $b0 = 'My::B'->new(16);
my $b1 = 'My::B'->new(6);

use Data::Dumper;
print Dumper A => $a0 - $a1;
print Dumper B => $a0 - $b1;
print Dumper C => $b0 - $b1;
print Dumper D => $b0 - $a1;