See 
PublishedAPI for packages intended to be used by Plugin and Contrib authors, or 
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See also 
Developing plugins, 
Developer's Bible, 
Technical Overview
This is a preference backend used to get preferences defined in a topic
and cache them in memory.
 ClassMethod new(@_) 
Creates a preferences backend object.
 ObjectMethod finish() 
Break circular references.
 ObjectMethod cacheHits() → $int 
returns the number of hits to the memory cache 
 ObjectMethod getEntry() → \%entry 
returns a cache entry for the given topic object
 ObjectMethod setEntry() → \%entry 
stores the private values into the global cache.
 ObjectMethod getModificationTime() → $timestamp 
returns the file modification time of the topic object 
 ObjectMethod invalidate($metaOrPath)  
this method can be called as an object as well as a class method.
If called as a class method the $metaOrPath parameter is mandatory
 ObjectMethod getCacheKey($metaOrPath) → $string 
this method is an object as well as class method, similar to invalidate().
it returns the key for the current object to cache its preferences for.