my $N = 100;
  my @file_b;
  tie @file_b, Tie::File, 'path/to/file_b' or die;
  my $last_match = 0;

  for my $line (<FILE_A>) {
      chomp $line;
      my $min = $last_match - $N >= 0 ? $last_match - $N : 0;
      my $max = $last_match + $N <= $#file_b ? $last_match + $N : $#file_b;

      my $pos = find(\@file_b[$min .. $max], $line);
      if ($pos >= 0) {
          $last_match = $min + $pos;
      } else {
          print "No match for $line\n";
      }
  }