sub find_between_tags($$$) {
  # pass me the string to parse, the start tag, and the end tag
  my $string_to_parse = shift;
  my $start_tag = shift;
  my $start_pos = index $stuff, $start_tag; # get string position of start tag
  return unless $start_pos > -1;            # return unless start tag found
  my $stop_tag = shift;
  my $stop_pos = index $stuff, $stop_tag;   # get string position of end tag
  $stop_pos > -1 ?                          # end tag found, return substring
    return substr $stuff, $start_pos,       # return substring
      $stop_pos - $start_pos + length $stop_tag : # including matching tags
    return substr $stuff, $start_pos, -1;   # return substring to end
}

print find_between_tags($stuff, "START", "STOP");

##</code><code>##

Benchmark: timing 100000 iterations of mine, original...
      mine:  4 wallclock secs ( 3.28 usr +  0.00 sys =  3.28 CPU) @ 30534.35/s (n=100000)
  original: 14 wallclock secs (13.12 usr +  0.00 sys = 13.12 CPU) @ 7622.53/s (n=100000)