RE: CGI::Fast and ithreads.

> >  $ perl -Mthreads -MCGI::Fast -e 'threads->new(sub{})->join()'
> >  Segmentation fault

The problem is with FCGI.pm (which is used by CGI::Fast):

> perl -Mthreads -MFCGI -e 'threads->new(sub{})->join()'
Attempt to free unreferenced scalar: SV 0x10105a30, Perl interpreter:
0x100102a8 during global destruction.
Segmentation fault (core dumped)

If you comment out the call to RequestX() (which goes into XS code)
inside Request() in FCGI.pm, the above runs without error.  These calls
do things with filehandles (and probably more) that obviously are not
thread-safe.