* Why would you want such a thing?

anonymous hashes and arrays are useful in some situations, for example yo can use a reference to a anonymous hash to store configuration of your program in a file, so you only require the file in the progam and you have the configuration, no need for parsing file you have all in native perl data structures; other use is when you are creating objects, an anonymous (again a reference) hash (or array sometimes) can be used to store the properties of the object; there are other uses but this ones are the ones i use more often.

* What's the scope on an anonymous thingie?

the same of the non anonymous, the block or the package depends on how you are you scoping them.

* Will they ever die/get lost/get clobbered

don't understand too well this question but i if you are asking if they are going to be deprecated i don't think so.

i hope this helps