To find the cost of the carpet, we need to first find the total area of the living room. The area of a rectangle is found by multiplying its length and width, so the area of Perry's living room is 9 yards * 10 yards = 90 square yards.
We know that the carpet costs $5.00 per square yard, so to find the total cost, we need to multiply the cost per square yard by the total number of square yards needed. This gives us a total cost of 90 square yards * $5.00/square yard = $<<90*5=450.00>>450.00.
Therefore, it will cost Perry $450.00 to buy enough carpet for his living room.