Did you look at the JSON docs?
"This module is a thin wrapper for JSON::XS-compatible modules with a few additional features. All the backend modules convert a Perl data structure to a JSON text and vice versa. This module uses JSON::XS by default, and when JSON::XS is not available, falls back on JSON::PP, which is in the Perl core since 5.14. If JSON::PP is not available either, this module then falls back on JSON::backportPP (which is actually JSON::PP in a different .pm file) bundled in the same distribution as this module."