gistfile1.txt

#!/usr/bin/perl

use strict;
use warnings;

use 5.010;

my $file_input = $ARGV[0];

open my $fh_read, '<', $file_input or die "can't open input file for reading";

my $halfway_point = int((split ' ', `wc -l '$file_input'`)[0] / 2);

my $backend_halfway_point_reached;

open my $fh_write_part1, '>', "$file_input.part1";
open my $fh_write_part2, '>', "$file_input.part2";

my $fh = $fh_write_part1;
while (<$fh_read>) {
  if (! $backend_halfway_point_reached && $. > $halfway_point) {
    $backend_halfway_point_reached = 1;
    $fh = $fh_write_part2;
    close($fh_write_part1);
  }

  print $fh $_;
}
close($fh_read);
close($fh_write_part2);