This must be one of those hundredth monkey things. I was just having a discussion with BrowserUk yesterday about lvalue closures. I was going to write it up and ask if anyone was actually using them for anything useful. Maybe I still will after I put some more thought into it.
The contrived example I was working with was a settable counter that would increment when accessed. Using an lvalue closure resulted in nicer syntax than a closure which returns a reference. Returning a reference allows the value to be settable but it also requires dereferencing when you just want to use the value and that's pretty yucky. Of course, there are other ways to do it too, such as a tied scalar.
It seems to me that, although really neato, it doesn't provide a whole lot outside of some slimmer syntax. I think the cases where the syntax is significantly leaner (such as your $foo->bar =~ s/foo/bar/ example) are probably pretty rare.
Given the experimental implementation of the lvalue attribute, I won't use it in production code. Right now, it's a toy to play with. I'm still undecided as to whether (or how often) I'd use it if it weren't experimental.
"My two cents aren't worth a dime.";