# This version adds difference counts:

open(FILE1, '<file1.txt') or die("Cannot open first file: $!.\n");
open(FILE2, '<file2.txt') or die("Cannot open second file: $!.\n");

$file1{$_}++ while (<FILE1>);
$file2{$_}++ while (<FILE2>);

foreach (keys(%file1)) {
   if ($file2{$_}) {
      $diff = $file2{$_} - $file1{$_};
      if ($diff) {
         if ($diff < 0) {
            print("Found in first file $diff times more than in second file: $_");
         } else {
            print("Found in second file $diff times more than in first file: $_");
         }
      } else {
         print("Found in both files an equal number of times: $_");
      }
   } else {
      print("Found only in first file ($file1{$_} times): $_");
   }
}

foreach (keys(%file2)) {
   unless ($file1{$_}) {
      print("Found only in second file ($file2{$_} times): $_");
   }
}