The straw comprises a tube (11), a stopper (13) arranged in the tube and having a passage (35) extending through it, and an emptying needle (14) having a first portion (31) protruding from said stopper as far as a distal end (19) situated in said tube, and a second portion (32) situated in said passage (35) of the stopper, said needle and said stopper forming an insert (12) entirely disposed in said tube and having an internal conduit (24) fluidically connecting the internal spaces of said tube that are situated to either side of said insert, said internal conduit being formed at least in part by the needle. The assembly comprises the straw and a device configured to make an opening in the tube. The method comprises the steps of establishing a fluidic communication between the needle and a receptacle, then of making an opening in the tube in order to connect said dose of liquid-based substance to the atmosphere and allow it to flow.
