 This randomized phase II trial studies how well cisplatin and fluorouracil work compared with carboplatin and paclitaxel in treating patients with anal cancer that cannot be removed by surgery, has come back at or near the same place as the primary tumor, or spread to other places in the body. Drugs used in chemotherapy, such as cisplatin, fluorouracil, carboplatin and paclitaxel, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. It is not yet known whether cisplatin and fluorouracil are more effective than carboplatin and paclitaxel in treating anal cancer. 