That's a lisp vector representation which (e.g.) emacs lisp and clojure use; I believe CL uses a #(1 2 3) reader format instead though so that's probably why Data::SExpression doesn't grok it (then again I don't know if offhand that module'd like the #(1 2 3) notation either . . .)

Considering that file's basically a serialized list of package-decr instances I'd probably agree with the suggestion to manipulate it from lisp instead. If you expounded on what you're trying to do with it you might prompt better suggestions, but so long as you've called package-initialize and Emacs has populated it you can monkey with things like this. Theoretically you could build up the info you wanted to export and then use json-encode to write out a more convenient representation.

(require 'cl) ;; Newer emacsen don't need this
(cl-loop for (pkg-sym pkg) in package-archive-contents
  collect (list pkg-sym
                (package-desc-archive pkg)
                (package-desc-version pkg)))
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