Sentence:  In systems with perfect-based strategies of expressing evidentiality this contrast is usually realized only in the forms of the past tense of the verb (<WSD>perfectoid</WSD> evidential systems may over time undergo an evolution into a full-fledged evidential system, but such an evolution presupposes a considerable reorganization of the entire verbal paradigm). 
Meaning of the word marked with <WSD> tags:  Derived from the perfect tense or maintaining an additional syntax for marking the perfect tense.