It is mostly used in legacy code as answered before.
If you are familiar with awk, it is the *way* to emulate multi-dimensional hash in awk/ and perl without reference
I admit I've used a bit in some places where the use is well hidden and localized.
You should prefer proper multi-dimensional hash (if it's really what you need).
imagine to get keys of hash{foo} in emulated hash version, it'd be very difficult to do so.
# with proper multi-dimensional hash
keys(%{$hash{foo}}); # get keys of hash{foo}
