FIG. 1 is a perspective view of a curling iron showing a new design; FIG. 2 is a top view thereof; FIG. 3 is a right side view thereof, the left side view being a mirror image; FIG. 4 is a front view thereof; FIG. 5 is a rear view thereof; and, FIG. 6 is a bottom view thereof. The broken lines are shown for illustrative purposes only and form no part of the claimed design.