The opinion-page column "Corporate Perestroika," Aug. 6, offers an innovative suggestion to "large US corporations like General Motors and IBM": that they create internal markets by allowing subsidiaries to do business with each other as separate, independent entities.The author fails to note that GM already implements this excellent principle; its wholly owned subsidiary, Electronic Data Systems, provides in- formation technology services to non-GM customers as well as GM internal customers. EDS, with its own CEO, its own corporate headquarters, and its own publicly traded stock, earns about 45 percent of its revenue from contracts outside General Motors. Craig Osborn, Seattle Letters are welcome. Only a selection can be published, subject to condensation, and none acknowledged. Please address them to "Readers Write," One Norway St., Boston, MA 02115.