It is not clear to all that this has to be specified as
-Accflags=-D... when used on the Configure command line.
Under ithreads the optree is read only. If you want to enforce this, to
check for write accesses from buggy code, compile with
-C<-DPERL_DEBUG_READONLY_OPS> to enable code that allocates op memory
+C<-Accflags=-DPERL_DEBUG_READONLY_OPS>
+to enable code that allocates op memory
via C<mmap>, and sets it read-only when it is attached to a subroutine.
Any write access to an op results in a C<SIGBUS> and abort.