Although your plants may look damaged now, they're not really in serious condition-so there's little reason to worry. The elongation and yellowing of stems and leaves is called etiolation. After a few days in the sun, your perennials should be just fine. They will color up quickly and grow normally. In the future, check for growth of perennials under the mulch a bit earlier. If left in place too long, winter mulch can smother the emerging perennials. It can also hold in excess moisture, causing them to rot.