use strict;
use warnings;
use autodie qw(open);

my @handles;
open $handles[0], '>>', '0_999';
open $handles[1], '>>', '1000_1999';
open $handles[2], '>>', '2000_2999';
$/="//\n";
while (<>) {
   print { $handles[int(($. - 1)/ 1000)] } $_;
}