That they did not tell anyone about. The whole point of a Dead Man's Switch, is that others know about it.
'Why keep it a secret?' Right from Dr Strangelove
Which is a classic case of mirror imaging: assuming that the logic with which the Soviets used in building such a device is the same we would have. But Perimetr is a great illustration of how that isn't the case. The Soviets didn't build Perimetr to deter the west, rather they built Perimetr to reassure themselves that even if the Americans manage to decapitate the Soviet leadership, the strategic rocket forces would still receive the order to retaliate. This would disincentive the Soviet leadership from trying to rush a decision, reducing the possibility of accidental war due to computer glitch or faulty intelligence. Once one considers that the purpose of Perimetr was really to prevent doddering old men like Andropov or Brezhnev from triggering a nuclear war in a panic, the secrecy starts to make a great deal more sense.
The unfortunate flip-side to this, however, is how it upset NATO plans for a decapitation nuclear strike on the Soviet leadership as a final attempt to prevent an all out nuclear exchange. Once Perimeter was active, all the destruction of the normal command channels in such a strike would have accomplished would have been to guarantee a massive "fail deadly" retaliation, with no "nuance" or "steps of escalation" or any of the other ways to control the crisis that the living leadership might have attempted. Basically, it would have guaranteed the nuclear apocalypse that the initial decapitation strike attempted to prevent.
In any case, unlike with Perimetr, we have it on the record that the Soviet intention was to announce the presence of the Cuban missiles once they were fully deployed. Hence, as noted it is impossible for the missiles to remain secret all the way into the 1990s, although American reaction to being presented with the weapons as a fait accompli rather then discovering it when the build-up was only partly completed, is interesting to speculate.