It is exactly as you said. The "activesheet" property has no meaning outside of Excel itself. It is the worksheet that is "on top" on your screen and where you can interact with. In Excel::Writer::XLSX that has no meaning, since you have to explicitly choose the worksheet you are working on. You can store that information in any variable and pass that variable to any subroutine you have written. Handing the whole workbook to your subroutine and relying on some (hidden) "activesheet" property to know on which worksheet it needs to work, seems not a good idea to me.

If your subroutine also needs to know in which workbook that worksheet resides, hand it a reference to both the workbook and the worksheet (i.e. the wokbook obkject and the number of the worksheet). As it is impossible to access a worksheet without knowing in which workbook it resides, you will always know from which workbook each worksheet comes. Just save that information in an array or hash