Is the problem that the workbook doesn't close or that the excel application doesn't close? You say you can tell that a certain object is still open -- can you explain how you determined that?

My first suggestion would be to undef $WB; as shown in the perldocs. $WB->close(); may not behave as you expect it to.

Also, in my code I use $book->close(0);. If memory serves, the parm is to indicate the workbook should not be saved, but I'd have to track it down again to be sure.

Update: I should say that I use $book->close(0); and undef $book;.