Just think up a name. But (from MSDN Library):

The names of event, semaphore, mutex, waitable timer, file-mapping, and job objects share the same name space. If you try to create an object using a name that is in use by an object of another type, the function fails and GetLastError returns ERROR_INVALID_HANDLE. Therefore, when creating named objects, use unique names and be sure to check function return values for duplicate-name errors.

One way to handle detecting an event firing is to poll using $event->wait([$timeout]). If you set the timeout to 0 the wait returns immediately with one of:

   +1    The object is signalled
   -1    The object is an abandoned mutex
    0    Timed out
[download]

See the Win32::IPC documentation and Win32::Event documentation.