We should write the expected value next to the actual result:
In order to make it obvious what are the expected values first we have to compute
them ourself - or bring in the expert, or the client who knows what she expects
from the application to display - and make sure the expected values are always written
next to the actual results.
That way it will be obvious to any tester what values need to be compared.
use FindBin qw($Bin);
use lib "$Bin/../lib";
print sum(1, 1), " 2\n";
print sum(2, 2), " 4\n";
print sum(2, 2, 2), " 6\n";