An opening mechanism for use with a fluid filled collapsible container is disclosed. The mechanism comprises an outlet spout adapted to be joined to a wall of the collapsible container, a perforator for opening the collapsible container through the outlet spout and a driver having a passage therethrough. The driver is received within the outlet spout and screw threadedly engaged therewith for advancement by relative rotation thereof to drive the perforator though the outlet spout to thereby open the collapsible container and permit fluid to be dispensed through the passage. A circumferential flexible seal is provided within the outlet spout on one of the driver and the outlet spout to engage with the other of the driver and the outlet spout to reduce leakage of fluid between the outlet spout and the driver as the collapsible container is opened.