This Perl script will read the binary skeleton format used by Spine v3.1.08 and output a JSON file that can be read by Spine v3.5. I have no plans to extend this with support for other versions, and it's not exactly pretty. Improvements are welcome.
Several sections are not currently being decoded because I didn't run into them in any of the files that I needed to convert. I will add support for them if I get a skeleton file that uses them.
skel2json.pl <file1.skel file2.skel file3.skel ...>
The script will convert the given .skel files and write .json files with the same file name. The last extension will be replaced with .json; for example, filename.skel becomes filename.json, filename.skel.bytes becomes filename.skel.json, and so on.