we say "condamné à perpétuité avec une période de sûreté de x ... ans"
It means nobody can reduce the sentence for any reason during that time. Then, provided it's long enough as regard the age of the person...
If you want to differentiate between the sentence and the meaning implied, you could say:
la perpétuité signifie la condamnation à la prison à vie.
But I agree that usually it never comes down to a lifetime in jail but a given period of time called 'une peine de sûreté incompressible' which is not to be reduced but is a maximum.
Hope it helps.