=over =item study SCALAR X =item study At this time, C does nothing. This may change in the future. Prior to Perl version 5.16, it would create an inverted index of all characters that occurred in the given SCALAR (or L|perlvar/$_> if unspecified). When matching a pattern, the rarest character from the pattern would be looked up in this index. Rarity was based on some static frequency tables constructed from some C programs and English text. =back