perldoc perl5150delta
говорит:
$$ can be assigned to
$$ 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
$$ read‐only. (This change also allowed a bug to be fixed while
maintaining backward compatibility.)
$$
- это текущий идентификатор процесса, почему в мире вы его назначили?