Because Text::CSV_XS was there - in it's extended implementation - way before Text::CSV, which was a braindead pure-perl implementation. After some discussion the author of the current implementation and me, we decided that Text::CSV would best be implementing a wrapper of the two modules. Text::CSV_XS is, as the name already shows, an XS implementation, which needs an ANSI C compiler, which not everybody has. That is why there is a pure-perl implementation, as a fallback for those that need the functionality, but do not have compiling possibilities