Once an instance of a perlclass class has had its fields initialised, it seems to be intended that the identities of the field variables should never change. To reflect that, I reckon it should be marked SvREADONLY. This would immediately have one effect on core behaviour. Currently there's specific logic to prohibit reblessing any SVt_PVOBJ. I think the intent of that was to prohibit reblessing an object that is currently blessed into a perlclass class, but it's been implemented from the wrong end. Such logic ought to be tied to the class rather than the object representation type. But it's also quite unnecessary to implement such a restriction as a new feature of bless: it's already prohibited to (re)bless a read-only object. So making perlclass class instances SvREADONLY is a better way to restrict reblessing. (Not that we need such a restriction anyway: reblessing an object away from a class doesn't threaten the class's integrity.) So I reckon SvREADONLY marking of instances should replace the specific reblessing restriction. This issue intersects with the Perl bug that I recently reported, "initfields CV visibility causes mayhem" <https://github.com/Perl/perl5/issues/20956>, concerned with the order of operations around instance initialisation. That bug report came out of experimentation around what the core's intent was regarding field variable identities, for reasons related to the present suggestion. If SvREADONLYness is added for the reasons that I propose here, then the flag can be used to resolve that bug. The confusion illustrated by that bug provides additional motivation to use SvREADONLY: it would provide clear demarcation of when field variables are expected to stop changing, and would be a status that's easily checked by croaking or asserting code. Setting SvREADONLY to reflect an intended immutability of instance structure would also be useful for cooperation with code on CPAN, were anyone to write a module that uses the SVt_PVOBJ data type. Having the initfield op type check SvREADONLY in order to honour immutability would also help in this regard. Patches attached. -zefram