The pouch comprises a flexible bag (12) having an opening (12A) with its edge fastened to two opposite walls of a reinforcing sleeve (18) that projects to the outside of the bag. The pouch can adopt a flat configuration in which the walls of the sleeve are situated one against the other, and a utilization configuration in which the walls form a channel that holds the opening open. The first wall (20A) of the sleeve (18) presents a first longitudinal end strip (30A), while the second wall presents a sliding edge (32B) suitable for sliding against the inside face of the first longitudinal strip on the pouch passing from its flat configuration to its utilization configuration.