I prefer a line-by-line approach and create a new array each time a line starting with a number is encountered. I assume that the data fits into memory, otherwise sorting will be difficult:

use strict;
use warnings;

my @lines;
while(<DATA>){
        push @lines, [$1] if /^(\d+)/;
        push @{$lines[-1]}, $_;
}
print @$_[1..@$_-1] for sort { $a->[0] <=> $b->[0] } @lines;

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

Update: @lines should really be called @blocks