You could look at using Win32::API to monitor the activation of the screen saver using SystemParametersInfo with with an action of SPI_GETSCREENSAVERRUNNING to determine when the screensaver kicks in (assuming it is enabled) and use that as your que for doing whatever your going to do.

You'd probably want to use something like Win32::Daemon or Win32::Daemon::Simple to put your script in the background, though it is probably easier to just right-click and exit:)