They violate encapsulation. A normal mutator can check the supplied argument before setting the attribute it is protecting, an lvalue subroutine never gets that chance.

Hmmm, for some reason, I thought that you could check that value. I guess that will teach me to post past my bedtime :)

I guess deep down, I want them to be like C#'s properties, but alas.....

A tied scalar can check before setting. Of course, in your example, if you had tied the foo element, then you'd have encapsulation.

While dragonchild may be mis-understanding your idea, he is correct that tied variables are a pretty sizable performance hit, and are propbably better not used in this way, where a simple getter/setter combo will do.

Of course, even better OO practice is to design classes with the minimum of getter/setters in the first place. Personally I only create them if and when I need them, so that I am not tempted to let it all hang out for no reason.