You can use "QueryObjectType" for debugging.
Print the object type for "$ppt", and the object type of "$ppt->ActivePresentation" and check if they are what you expected.

Win32::OLE->QueryObjectType(OBJECT) 
   The QueryObjectType() class method returns a list 
   of the type library name and the objects class name. 
   In a scalar context it returns the class name only. 
   It returns undef when the type information is not 
   available.
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