There is a article in Sept 17th issue of Wired Magazine called Inside the Apocalyptic Soviet Doomsday Machine that is very interesting and a good read.
While most Doomsday Machines are announced as a deterrence, the Perimeter system was to have been keep a secret from the US. It was not meant as a deterrence to the US to attack the Soviets but as a deterience for the Soviet leadership to over react in the case of a possible attack.
The Perimeter system guaranteed that if the USSR leadership was destroyed by a preemptively strike there was a system in place had the ability to launch an automated respond of any nuclear weapons that were still functioning.
Apparently the Perimeter System is still up and functioning.