This is yet another bug based on parser error recovery screwing up stack
synchronisation. I've converted the segv into an assertion failure in
commit dce3f5c3fd788f1c2e451e3760f05a347c949eff, but the actual bug with
error recovery remains.
-zefram
Re: [perl #131631] Segfault in Perl_cv_undef_flags
by Zefram