Yes, unless you switch, your Pokemon is going to faint. This is why Perish Song is useless without Mean Look support, because your opponent is going to do the same. Take advantage of them going to try to switch by using Pursuit. This is a powerful strategy that can be used by Altaria, Honchkrow, Absol, and of course, Smeargle. If you want the Perish Song/Mean Look strategy, it can be learned by: Wigglytuff, Dewgong, Jynx, Gengar, Smeargle, Marowak, Lapras, Politoed, Honchkrow, Mismagius, Celebi, Altaria, Absol, Kriketune, and Arceus.