1) forget split ~ why split, parse, then join?

2) forget m// ~ when you have index

3) generalize for more tags

try this~

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 s
+tart 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 e
+nd tag
  $stop_pos > -1 ?                          # end tag found, return su
+bstring
    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");
[download]

Update:

thanks tachyon, for teaching me Benchmark!

here's the lowdown on original method vs. mine:

Benchmark: timing 100000 iterations of mine, original...
      mine:  4 wallclock secs ( 3.28 usr +  0.00 sys =  3.28 CPU) @ 30
+534.35/s (n=100000)
  original: 14 wallclock secs (13.12 usr +  0.00 sys = 13.12 CPU) @ 76
+22.53/s (n=100000)
[download]

~Particle