Is 'perforce' being used correctly in this sentence The fire alarm is so deafening that we must perforce evacuate the building?
No, perforce is not precisely correctly used in this sentence
because the fire alarm is the subject of the sentence. The noise of
the alarm might well indeed be unpleasant, but it alone would not
require the evacuation of the building. As poor an option as not
evacuating might be, it does remain an option. The alarm does not
physically require an evacuation.