$library = [contents of library.txt]
$input = [contents of input.txt]

my %in_library = ();
foreach $entry (split(/\n/, $library)) {
  $in_library{"$entry"} = 1;
}

my @input = split(/\n/, $input);

foreach $entry (@input) {
  if ($in_library{"$entry"}) {
    print "found - $entry\n";
  } else {
    print "new - $entry\n";
  }
}