$self->foo(123)      # chaining of mutator calls
     ->bar(456);
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this always gave me the heebies because returning the object for success seems a little fragile. What if ->foo() fails? Is it still sensible to return the object? What happens if you have a mutator (well, a not-mutator according to hardburn) that rejects the data presented for whatever reason? If your mutator returns an error code because ->foo(123) is bad input, you wind up with, i.e., "123 is an invalid foo"->bar(456) and your code goes pear-shaped.

Sure, maybe "123 is an invalid foo" is up for argument as a sensible error code. The point is that it seems like a way to limit your object if you need to do any interesting things to communicate failure to back to your user.