1. You can thread and have and have a semiphore set when the directory is changed.
2. You can fork, and check status.

There is no need for threading or forking. The wait() method takes a timeout in milliseconds.

So, set up a Tk->after callback that goes off every say 1/10 of a second and call

my $notifyObj = Win32::ChangeNotify( ... );
my $found = 0;
my $abandoned = 0;

sub checkNotify(
    my $rv = $notifyObj->wait( 0 );
    $found = 1 if $rv = 1;
    $abandoned = 1 if $fv = -1;
    die 'Notify error' unless defined $rv;
}

$mainWindow->after( 100, \&checkNotify );
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See Win32::IPC and Win32::Semaphore for details.