I tried running your code but found I didn't have JSON::XS installed under v5.24. On attempting to install using cpan, I got a message which in essence said:
"... standard perl versions 5.022 and up are not supported by JSON::XS."
That is interesting.
I was able to install it on Windows without any issues with Strawberry Perl's cpan and ActivePerl's ppm
The "panic: sv_chop %s" diagnostic was introduced in v5.12
I saw that too on Perldiag. Unfortunately, it only tells *what* is happening, not *why* it is happening...