My brother got orders to report to an appointment at Perth in the army. what is Perth?
Perth is the name of a certain place in Central Scotland. It was still the capital of Scotland until the assassination of James 1 in 1437. This means that your brother has been called in an official duty to that place.
There is also a place in Australia called Perth. The Army also uses a lot of acronyms, it could be an acronym for something.