If you want to keep your hash structure, here are 2 alternatives that do not lose data:

The first one uses the ad# as the key (inverse of your example).
The second one uses the percent as key, but stores the Ad#'s as Values in a HOA.

#First way - Ad# as key:
my $str='27:43:33:21:23:19:27:6';
my( $temp,%h); 
%h=   map {if($temp){
               my $y=$temp;
               $temp=undef;
               $_=>$y
           }else{
               $temp=$_;
               undef()=>undef;
           }
      }   split/:/,$str; 
delete $h{undef()};

print qq($_=>$h{$_}\n) for keys %h;

#Second way - using a HOA

my $str='27:43:33:21:23:19:27:6'; 
my( $temp,%h); 
for ( split/:/,$str){
   if($temp){
     push @{$h{$temp}},$_;
     $temp=undef; 
   }else{
     $temp=$_;
   }
};

print qq($_=> @{$h{$_}}\n) for keys %h;
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