$$can be assigned to
       $$was made read‐only in Perl 5.8.0.  But only sometimes: "local $$"
       would make it writable again.  Some CPAN modules were using "local $$"
       or XS code to bypass the read‐only check,so there is no reason to keep
       $$read‐only.  (This change also allowed a bug to be fixed while
       maintaining backward compatibility.)

$$是当前的进程ID,为什么要在世界上分配给它？