I have the same issue after upgrading from Strawberry perl 5.12.3 32 bit to perl 5.26.1 64bit. Had to ugprade because of SSL issues and way overdue upgrade of perl. my $rushhandle = Win32::GUI::FindWindow("tfmrush", ""); #this finds a valid handle Win32::GUI::SendMessage($rushhandle, WM_COPYDATA, 0, $copy_data_struct) ; # no activity occurs in FTPRush I am about to try the 32bit and see if that helps. I am on Windows 7, so i dont think the problem is specific to Win10