parn.pl:
use strict;
use warnings;

$| ++;

print "Entering parn.pl...\n";
my $chld;
open($chld, "perl -w chld.pl|") || die "failed to open child process\n";
my @ready;
while (1) {
    my $from_chld = <$chld>;
    print $from_chld * 2, "\n";
}
   
chld.pl:
use strict;
use warnings;

$| ++;

while (<>) {
   if ($_ < 100) {
      print "$_";
   }
}