use strict;
use warnings;

use IPC::Open3 qw( open3 );

for (my $size = 1024; ; $size += 1024) {
   print("$size\n");
   my $pid = open3(
      local our $to_chld,
      local our $fr_chld,
      local our $fr_chld_err,
      perl => (
         -e => 'print "x" x $ARGV[0]',
         $size
      )
   );
   waitpid($pid, 0);
}