why one would need it and the other would not
Well, I dont know why for sure, but maybe its do with what Perl users do more often.

I cant think that i've every dynamically written to ISA (reading it is a different story if you are doing multiple inheiretance.)

Whereas I have on MANY occasions dynamically added new subs to an existant class.

Just think of any code that gets autoloaded. If can cached then a lot of things would get confused wouldnt they? Or there would have to be a cache flush any time something gets added to the package level symbol table. Not very efficient.

Well thats my theory anyway.

Yves