The ornamental design for a decorative eyeglass, as shown and described.
FIG. 1 is a perspective view of the decorative eyeglass showing my new design.
FIG. 2 is a side view thereof in an alternate position.
FIG. 3 is a front view thereof as shown in FIG. 1; and,
FIG. 4 is a side view thereof in an alternate position.
The broken lines, within the claimed design, in FIGS. 3 and 4 that depict a person are for illustrative purposes only. None of the broken lines form a part of the claimed design.