while (<>) {
     if ($_ =~ m/\n{1}/) {
     }
     if ($_ =~ m/\n{2}/){
          s/\n{2}/\n/gs;
          print;
     }
     elsif ($_ =~ m/\n{3,}/){
          s/\n{3,}/\n/gs;
          print;
     }
}