And here's my stab at creating a Marpa-based parser, based on JSON::Decode::Marpa: https://github.com/perlancar/perl-SExpression-Decode-Marpa/. It's unfinished (its number and string rules, particularly, are still not adjusted), but can already parse the original archive-contents file, a bit faster than Data::SExpression:

% time perl -Ilib -MSExpression::Decode::Marpa=from_sexp -MFile::Slurper=read_text -E'from_sexp(read_text "archive-contents")'

real    0m4.023s
user    0m3.818s
sys     0m0.204s