use threads; #lets us make threads
use threads::shared; #lets us share variables between threads

my $job_done : shared = 0; #allow the main thread to notify the child thread it is done.

sub watcher {
	my $error = 0;
	until ($job_done) {
		if (error windows appeared) {
			$error = 1;
		}
		sleep 1; #use Time::HiRes qw(sleep) or select(undef, undef, undef, 0.5) if you need less than 1 second testing
	}
	return $error;
}

my $watcher = threads->create \&watcher; #start the watcher thread
system(call the program); #this will tie up the main thread until it finishes
$job_done = 1; #this should let the thread exit on the next loop
my $error = $watcher->join; #clean up the thread and get it's return.