Still, if the output is more complex than a single, short number, or if there are many results, it will be difficult to the person comparing to notice the differences. What if we have 100s of test cases?

examples/perl/tests/t04.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";
print sum(3, 3), "  6\n";
print sum(4, 4), "  8\n";
print sum(5, 5), "  10\n";
print sum(6, 6), "  12\n";
print sum(7, 7), "  14\n";

Output:

examples/perl/tests/t04.pl.out
2  2
4  4
4  6
6  6
8  8
10  10
12  12
14  14