The ornamental design for a pack with opening, as shown and described.
FIG. 1 is a perspective view of a pack with an opening according to the new design;
FIG. 2 is a front view of the pack;
FIG. 3 is a back view of the pack;
FIG. 4 is a first side view of the pack;
FIG. 5 is a second side view of the pack;
FIG. 6 is a top view of the pack;
FIG. 7 is a bottom view of the pack;
FIG. 8 is a front view of the pack disposed on a human torso; and,
FIG. 9 is perspective view of the pack, wherein the pack is partially open.
The broken lines illustrating a human torso in FIG. 8 represent environment. The remaining broken lines illustrate portions of the pack with opening. None of the broken lines form part of the claimed design.