#!/usr/bin/env perl

use strict;
use warnings;

my @data;
map { /^\d+/ ? push(@data, $_) : ($data[-1] .= $_) } <DATA>;
print sort { ($a =~ /^(\d+)/)[0] <=> ($b =~ /^(\d+)/)[0] } @data;

__DATA__
2 [...]
  [line1]
  [line2]
  [line3]
3 [...]
  [line1]
1 [...]
  [line1]
  [line2]
2 [...]
  [line1]
  [line2]

##</code><code>##

1 [...]
  [line1]
  [line2]
2 [...]
  [line1]
  [line2]
  [line3]
2 [...]
  [line1]
  [line2]
3 [...]
  [line1]