When the Saruman Falls to Corruption plot card is played is Isengard then permanently perilous or perilous only as long as it is an active plot.

Thematically and by strict reading of the card I believe it is permanently perilous.

I agree with what David said on BGG, only Perilous while the Plot is in play.

The effect only lasts while the card is in play.

If you need a "thematic" reason, just think of it this way: As we re-write "history" through the course of the game, Sauron pushed Saruman over the edge a bit sooner than in the books, the good guys spotted this, and stopped this plot before Gandalf was imprisoned.

If you need "game" reason: there are no effects of any other plot cards which persist after the plot is removed.

TK