Why doesn't perl use the global USE flag "threads" instead of "ithreads"?
Because they aren't related for starters. threads in use.desc is generally speaking for pthreads. ithreads is for iterated threads within perl and should only be used by those who know what they're asking for. A perl capable of running in threaded (ithreads that is) mode is in no way guaranteed to link or work with software that usually supports perl. However, blead edge aside, there are folks that use gentoo that use perl that require/need the ithreaded layer for their work. Hence the flag (and why its not enabled by default).
Thanks for the explanation! use.desc describes the global threads flag as being "usually pthreads" which leaves room for other options, hence my query. However, I didn't realise perl has multiple options for threading of which ithreads is just one.
