Why would you assign to $$?

perldoc perl5150delta says:

   $$ can be assigned to
       $$ 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.)

$$ is the current process ID, why in the world would you assign to it?