I've tried it as well, with the same results. This may be a bug. Perhaps if more experienced XS Monks have a look and confirm, you could open a ticket.

I thought if I looked through the test files for an example, that'd help, but there don't seem to be any tests at all for this feature:

$ cd JSON-Parse-0.40
$ grep -r "warn_only" *

Json3.xs:warn_only (parser, onoff)
Json3.xs:    parser->warn_only = SvTRUE (onoff) ? 1 : 0;
json-common.c:    unsigned int warn_only : 1;
json-entry-points.c:    parser_o.warn_only = 1;
lib/JSON/Parse.pod:=head2 warn_only
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Relevant code in Json3.xs:

void
warn_only (parser, onoff)
    JSON::Parse parser;
    SV * onoff;
CODE:
    parser->warn_only = SvTRUE (onoff) ? 1 : 0;
[download]