Nice, gentle intro - good job!

A couple of easy-to-fix tips :

• The link in your note above is not click-able. Here is a Clickable version of your http://earth.prohosting.com/foreach/perl/oo.html
• In your 'add' and 'subtract' methods, your return values though correct, are not explicit. I would rather see the code written as

    $self->{num} += $add;
    return $self->{num};
  [download]

  Yes, that could be single-ligned like
  return $self->{num} += $add;
  But that could potentially cause brain-damage for confused newbies.