But what went wrong?

• We need to be more verbose

#!/usr/bin/perl -w

use strict;

use Test::Simple tests => 2;
use MyMath;

my @tests = (
  [0 => 1],
  [6 => 120],
);

foreach (@tests) {
    my ($in, $out) = @$_;

    my $res = MyMath::factorial($in);
# more verbosity
    ok($res == $out, "factorial($in) gave $res, expected $out");
}

$ MyMath.t
1..2
ok 1 - factorial(0) gave 1, expected 1
not ok 2 - factorial(6) gave 720, expected 120
#     Failed test (MyMath.t2 at line 16)
# Looks like you failed 1 tests of 2.