So Let's Start From the Beginning

• We read Test::Tutorial, as recommended...
• ...and start with Test::Simple
• Enters MyMath.t

#!/usr/bin/perl -w

use strict;

use Test::Simple tests => 2; # <- Number of tests
use MyMath;

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

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

# ok(boolean result, test description)
    ok(MyMath::factorial($in) == $out, "$in! = $out");
}

$ MyMath.t
1..2
ok 1 - 0! = 1
not ok 2 - 6! = 120
#     Failed test (MyMath.t1 at line 15)
# Looks like you failed 1 tests of 2.