my ($pin,$pout,$cin,$cout,%kid);
{
    pipe $pin, $cout;
    pipe $cin, $pout;
    my $cpid = fork;
    die $! unless defined $cpid;
    $kid{$cpid} = 1, last if $cpid; # parent
    close $pin or die $!;   # in child to end of block
    close $pout or die $!;
    while (<$cin>) {
        # do filter-like stuff to $_
        print $cout $_;
    }
    exit 0;
}
# parent
close $cin or die $!;
close $cout or die $!;
# print to $pout and read from $pin, maybe in a select loop
# depends on expected behavior of the child.
delete $kid{wait()} while %kid;