I think the point here is that you'd have to do something like this

  $x = new Number(1)
  $y = clone $x;
  $y->absolute();

to work with the absolute value of variable $x without actually changing $x

Yes, this is a common source of bugs when you're not sure what is changing. See below, someone suggests to use 'to' and 'as' prefix to convey immutable / mutable methods.