If you want, you can read alternately in the parent and the child.

(Using sysread because it is unbuffered.)

#!/usr/bin/perl

# https://perlmonks.org/?node_id=1230099

use strict;
use warnings;
$| = 1;

my $me = fork() ? 'parent' : ' child';

while( sysread DATA, $_, 1 )
  {
  print $me eq 'parent' ? uc : lc;
  select undef, undef, undef, 0.1;
  }

1 while wait > 0;

__DATA__
one
two
three
four
five
six
seven
eigth
nine
ten
[download]

Outputs (on my machine) :

OnE
TwO
ThReE
FoUr
fIvE
SiX
SeVeN
EiGtH
NiNe
tEn
[download]

Posted because I think it's funny :)

EDIT: fixed incomplete output paste.