use threads;
use threads::shared;

$| ++;

my $line = "";
share($line);

sub read_stdin {
    while (<STDIN>) {
        lock($line);
        $line = $_;
        print "child read in: $_\n";
    }
}

threads->create(\&read_stdin);
while (1) {
    if ($line ne "") {
        lock($line);
        print "parent process $line\n";
        $line = "";
    } else {
        print "do something else\n";
        sleep(1);#this nothing but to slow down the prints
    }
}