In my experience, the best way to approach a proprietary file in the Windows environment is to employ OLE.   Whether the document in question is encrypted or not.

To my way of thinking, when you do this you are going straight to the actual source of the document:   this is the software that built the thing, and this is the interface that was devised by the company that designed that software.   Sure, there’s some inefficiency involved, but typically we have computing resources to spare even on a lowly desktop machine.   Perl has a fine OLE interface module.   The question is how thoroughly the vendor approached the design and implementation of their OLE interface.   In Microsoft’s case, it will be very good because, after all, OLE was their idea in the first place.