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.

examples/perl/tests/t03.pl
use strict;
use warnings;

use FindBin qw($Bin);
use lib "$Bin/../lib";
use MyTools;

print sum(1, 1),    "  2\n";
print sum(2, 2),    "  4\n";
print sum(2, 2, 2), "  6\n";

Output:

examples/perl/tests/t03.pl.out
2  2
4  4
4  6

Now it is better.