This runs a tiny bit faster for me: (5% on short strings, much less for longer strings.)

sub substrings {
  my $string = shift;

  my @result = ();

  foreach my $start (0..length($string)-1) {
    my $substr = substr($string, $start);
    while (length($substr)) {
        push @result, $substr;
        chop($substr);
    }
  }

  return @result;
}
[download]

But forget speed. The fun solution is:

sub substrings {
  local $_ = shift;
  my @result;
  do { push @result, /(?=(.+)$)/sg; chop } while (length);
  return @result;
}
[download]

Now if I could only figure out how to do it all in one regex... anyone?

Update: That durn mjd got there before I even submitted this... though he used embedded code. Is there any way without it?