# this makes the fork and pipes STDOUT of child to OUTPUT of parent.
my $pid = open(OUTPUT, "-|");

if($pid == 0)
{
    # we are in child now. To provide input I need another fork.
    # This time the STDIN of child is piped from INPUT of this process.
    my $another_pid = open(INPUT, "| another_script_or_program");
    # now check the return value and pass the input thru INPUT
    print INPUT "my input\n";
    exit(0);
}

# the main process just reads data from OUTPUT
print while(<OUTPUT>);

# do waitpid
close(OUTPUT);