# s/(?<=[a-zA-Z])([12])/ $1 == 1 ? 2 : 1 /eg;
   # s/([a-z])([12])/ $1 . ($2 == 1 ? 2 : 1) /eig;
   s/(.)([12])/ $1 . ($2 == 1 ? 2 : 1) /eig; 
   
this might work:
   s/([A-Z])([12])/ $1 . ($2 == 1 ? 2 : 1) /eig;