# A version that also checks for lines in file2 that are not in file1:

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

%file1 = map { $_ => 1 } <FILE1>;
%file2 = map { $_ => 1 } <FILE2>;

foreach (keys(%file1)) {
   if ($file2{$_}) {
      print("Found in both files: $_");
   } else {
      print("Found only in first file: $_");
   }
}

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