{
  my @cache; # a cache of substring-finding subs
  sub substrings {
    my $string = shift;
    my $length = length $string;

    # use cached sub if we have one
    return $cache[$length]->($string)
      if exists $cache[$length];

    # create sub to find substrings for this length
    my $sub = 'sub { $_ = shift; return (';
    foreach my $length (1..length($string)) {
      foreach my $offset (0..length($string)-$length) {
        $sub .= "substr(\$_,$offset,$length),";
      }
    }
    $sub .= ")};";
    $cache[$length] = eval $sub;

    # and use it
    return $cache[$length]->($string);
  }
}