Would I be correct in supposing that this barley is similar to pearl tapioca? (See second body paragraph.) If so, then the name most likely has had - at some point - something to do with the process. Specifically, this tapioca (or barley) has undergone the pearling process, so it can now be referred to as pearled to use the participle, and also pearl from describing its new shape.

It's just a thought, and although it doesn't really sway the issue, it has made me quite hungry for some tapioca...and possibly some of your soup, Father!