my @values = <DATA>;
   print "\nValues:\n", @values;
   chomp @values;

   sub max_len
   {
      my $len = -1;
      for ( @_ )
      {
         $len < length $_ and
         $len = length $_;
      }
      $len
   }

   sub min
   {
      $_[0] < $_[1] ? $_[0] : $_[1]
   }

   my $max_length = min( 10, max_len(@values) );


   # normally, we only want to allow valid ID characters:
   my @idents = map { s/\W+/_/g; $_ } @values;

   print "\nIdentifiers (raw):\n", map "$_\n", @idents;

   print "\n\n\n\n";


   my %mapping; # contains the final result

   { # this is the part that does the real work:
   my %h;
   for ( @idents )
   {
      push @{ $h{ substr $_, 0, $max_length } }, $_;
   }
   for ( keys %h )
   {
      my $n = @{ $h{$_} };
      my $n_width = length $n;
      if ( $n == 1 )
      {
         $mapping{$_} = $h{$_}[0];
      }
      else
      {
         my $prefix = substr $_, 0, $max_length - $n_width;
         my $oneup;
         for ( @{ $h{$_} } )
         {
            my $k;
            do {
               $k = $prefix . sprintf '%0'.$n_width.'d', ++$oneup;
            } while exists $mapping{$k};
            $mapping{$k} = $_;
         }
      }
   }
   } # end of main algorithm.

   for ( sort keys %mapping )
   {
      print "$_ => '$mapping{$_}'\n";
   }

   __DATA__
   line one
   line two
   lines end here not
   lines end here too
   yyyyyyy
   yyyyyyy
   xxxxxxxxxx
   xxxxxxxxxx
   zzzzzzzzzzzzz
   zzzzzzzzzzzzz
   r
   r
   rr
   rr
   r1
   r1
   r12
   r12
   Four score and seven years ago, our fathers...
   Four score and seven years ago, our mothers...
   Four score and seven years ago, our brothers...