use Win32::API;
my $GetWindowThreadProcessId = Win32::API->new('user32', 'GetWindowThreadProcessId', ['N', 'P'], 'N');
my $process_id = pack 'V', 0;  # buffer of 4 bytes
my $thread_id = $GetWindowThreadProcessId->Call($hwnd, $process_id);
$process_id = unpack 'V', $process_id;