This is the "system PROGRAM LIST" syntax. It's documented more fully under exec. There are some cross-references in the system page, but I can see how you could easily miss them if you weren't looking out for something like that. Maybe the documentation could usefully be expanded to add an example of this syntax.

(pc88mxer's version actually contains a syntax error, BTW. There shouldn't be a comma after the curlies -- it's an indirect argument, like the filehandle in print.)