Perle is a German hop that is traditionally used for Bittering. Liberty is an American hop that has German Noble Hallertaur in its lineage.
The Liberty will impart more hop aroma if used late in the boil. However, a good Germen Hefeweizen recipe will only use bittering hops anyways, with no need for hop aroma or flavor.
If you can't find either of those, any of the following will be just fine in your beer:
Magnum (though you would need a lot less - Magnum is packed with Alpha Acid)
The recipe just calls for 1 oz of either Perle or Liberty. The intent of that recipe is to try to keep the target IBUs to between 16 and 20 IBUs. All that means is 1 oz of 4% AA hops. Use more if you buy hops with lower stated %AA. Use less if the hops you buy have higher stated %AA.
Ask here once you have the actual hops in your hands, tell us the actual varietal and %AA you bought and we can help you figure out exactly how much to add to stay in that range.
Good Luck and welcome to the hobby-