package Package; {

    sub perform {
        return $_[0] + $_[1];
    }
}

package main; {
    use Mock::Sub;
    use Test::More;

    tests();

    my $m = Mock::Sub->new;
    my $changed = $m->mock('Package::perform');
    $changed->return_value($_[0] - $_[1]);

    tests();

    done_testing();

    sub tests {
        is Package::perform(5, 5), 10, "5 + 5 = 10";
    } 
}