The ornamental design for a holder for a portable electronic device, as shown and described.
FIG. 1 is a perspective view of a holder for a portable electronic device showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is a perspective view thereof with the ring twisted; and,
FIG. 9 is a perspective view thereof with a portable electronic device encircled by the ring.
The dash-dash lines depict portions of the holder for a portable electronic device that form no part of the claimed design. The dot-dot-dash lines showing the portable electronic device illustrate unclaimed environment and form no part of the claimed design.