I assume your snippet is intended to go inside the OP's for loop:

foreach my $i ( 0..$#data ) {
   my %keys = keys %dataPos; # Changed  hash name to match hash OP is 
+building
   my $hkey = $data[$i];     # Changed data lookup to array to match O
+P code.
   if ((my @found = grep /^$hkey$/, %keys) > 1) {
      $hkey .= '_';
   }
   
   $dataPos{$hkey} = $i;
   
   print "$data[$i] -- > $i\n";        
}
[download]

After I tweaked your code to work with the OP's code, there are still problems. You have a unnecessary nested loop that will grow with each pass through the outer loop. Also, if a key shows up three or more times, the last value will overwrite the value stored for the second key.

I've fixed these issues below.

foreach my $i ( 0..$#data ) {
    my $key = $data[i];
    
    # append '_' until no matching key exists.
    while ( exists $dataPos{$key} ) {
        $key .= '_';
    }
   
    $dataPos{$key} = $i;
    print "$key -- > $i\n";        
}
[download]

Even after its been fixed, I still wouldn't use this approach. I'd store repeated values in arrays as other posters have suggested.