I wouldn't call eLisp a DSL. True, it has a bunch of domain specific commands but is still a GPL (General Purpose Language).

That's pretty much like with JS which has DOM specific commands but can still be used for other stuff (actually eLisp was the role model for all those embedded scripting languages° - Brendan Eich was originally hired by Netscape to create a Scheme like language)

The best example for an (external) DSL is SQL. It is limited to a specialized domain, e.g. you will rarely see computer games realized entirely in SQL.

But if you are an SQL expert you can contribute to Perl, Python or PHP projects, without knowing much of the host language.

The case of this thread - Rex - is a configuration DSL realized "internally", i.e. by reusing language elements of the "host language" Perl. (SQL snippets are external strings)

You don't need to know much Perl to be able to use this DSL.(pretty much like SQL-expert can patch SQL code in a Perl project)

The advantage to JSON:

• to catch typos or grammatical errors. In JSON you'd need to write a parser which descends the data structure (Kind of reinventing a DTD). But Rex's DSL the commands must be predefined and can validate the input immediately
• you can extend this DSL to your needs by adding your own subs
• it's more expressive, because Perl offers much more syntactic sugar than nested hashes/arrays have

°) I only mentioned LISP because DSL's where invented there, especially with the use of reader macros* which bend the parser by overriding single symbols. For instance you can make LISP parse embedded JSON

*) not to be confused with LISP's syntactic macros or even C's preprocessor macros.