sub score{
  my($str, $array) = @_;
  # ensure that longer string comes before its prefix
  my @substring = sort { $b cmp $a } @$array;
  my $re = join '|', map "(?=($_))", @substring;
  my($count, $next) = (0, 0);

  while ($str =~ /$re/g) {
    # $-[0] is the position at which we matched
    # @- describes the matched captures, so $#- is the actual capture matched
    my($start, $which) = ($-[0], $#- - 1); 
    my $end = $start + length($substring[$which]);
    $next = $start if $next < $start;
    next if $end < $next;
    $count += $end - $next;
    $next = $end;
  } 
  return $count;
}