#!/usr/bin/perl -w

use strict;

use FileHandle;

use constant BUFSIZE => 12000;
use vars qw($buf);
$buf = "A" x BUFSIZE;

STDIN->setvbuf($buf,_IOFBF,BUFSIZE)
  or die "Couldn't setvbuf\n";
while (<>)
{
  print "This is a line.\n";
}

##</code><code>##

$ strace -e read -f perl /tmp/t101 </etc/passwd
...
read(0, "root:x:0:0:root:/root:/bin/bash\n"..., 12000) = 1482
This is a line.
...
read(0, "", 12000)                      = 0