in reply to Re^8: Shouldn't references be readonly? (updated)
in thread Shouldn't LITERAL references be readonly? (updated)

Note that assigning to certain "unassigned scalars" is legitimate, such as the one returned by substr. Unassigned scalars with set magic simply wouldn't be made read-only, of course.

But you know what else wouldn't be made read-only? "Unassigned scalars" from XS code. Making unassigned scalar *created by Perl* read-only would create inconsistencies with those created by XS code. This reduces the benefits of making any change.