a-la-Class::Struct, with exported functions that install accessors by means of eval

Perhaps, but Class::Struct may not be the best module to base your OO interface on since it's intent is to implement C structs in Perl. That and eval is evil ;)

using subroutine attributes, and perhaps variable attributes (still experimental in 5.8?)

It'd certainly look very nice syntatically, but I believe attributes are still somewhat of a grey area (esp. for variables).

installing methods on the fly using AUTOLOAD and *{$AUTOLOAD} = sub { do { this() and that() } } ;

This is certainly a nice way of generating accessor methods if somewhat heavy-handed (*anything* could be an accessor).

Unfortunately there is a fundamental flaw to this whole approach - it is *impossible* to have truly encapsulated classes in Perl without taking advantage of lexical scoping. So while closures may look like an abberation in your otherwise pristine code, it may be a necessity to achieve your goal of true encapsulation. I think I'll write a meditation on the whole subject of encapsulation in perl in the next week or so.
HTH

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broquaint