#!/usr/bin/perl

# http://perlmonks.org/?node_id=1213573

use strict;
use warnings;

#print join('', <DATA>) =~ /^start.*\n(?:\+.*\n)*/gm; # my preference 
+:)

while(<DATA>)
  {
  /^start/ .. /^(?!start|\+)/ and /^(start|\+)/ and print;
  }

__DATA__
ignore this line :)
start text
+ 123
+ 456
+ 789
some other text not starting with +
another test case
start text
+ foo
+ bar
start text
+ one
+ two
+ three
also ignore this line
[download]

Outputs:

start text
+ 123
+ 456
+ 789
start text
+ foo
+ bar
start text
+ one
+ two
+ three
[download]