If the file is not too large to read into memory you can slurp the whole thing and then split into "records" at points preceded by a line terminator and followed by a digit. You can then sort using a Scwhartzian Transform.

$ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<EOD or die $!;
2 [...]
  [line1]
  [line2]
  [line3]
3 [...]
  [line1]
1 [...]
  [line1]
  [line2]
2 [...]
  [line1]
  [line2]
EOD

my $input = do {
   local $/;
   <$inFH>;
   };
close $inFH or die $!;

print for
   map  { $_->[ 0 ] }
   sort { $a->[ 1 ] <=> $b->[ 1 ] }
   map  { [ $_, m{\A(\d+)} ] }
   split m{(?<=\n)(?=\d)}, $input;'
1 [...]
  [line1]
  [line2]
2 [...]
  [line1]
  [line2]
  [line3]
2 [...]
  [line1]
  [line2]
3 [...]
  [line1]
$
[download]

I hope this is helpful.