As I recall, this is one situation where the computer cheats. Against a human player, they won't have the option of switching out until they start a turn with your Lapras not on the field; so you can safely switch out of the last turn of Perish Song, leaving them to be trapped and die. Against a computer player, if you switch that last turn, they'll take their action as though you've already switched and they'll also switch out. At least, I'm pretty sure I read somewhere that's what happens. They might of course have fixed it for B&W; you could ask someone to test it in the research thread...