Minor tweak:

  $knob = $_[0] if @_;
[download]

That way, Parent->knob() is an accessor, while Parent->knob(undef) is a mutator that sets the $knob to undef. Slightly better:

  if (@_)
  {
    # check that $_[0] is allowed.
    $knob = $_[0];
  }
[download]

but that's only if there are invalid values you want to block.