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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
 ClassMethod new( $iter, $sub, $data ) 
Construct a new iterator that will filter $iter based on the results from
$sub. $sub should return 0 if the next() from $iter should be filtered and
1 if it should be treated as the next item in the sequence.
$data is an optional arbitrary data item which will be passed to $sub in $_[1]