JSON::PP has been core for quite a long time now, the issue you've raised is in the right place. I'm pretty sure Kenichi is correct, and that this is related to an old version of the module:

marto@Shemp:~$ perl -MJSON::PP -e 'print $JSON::PP::VERSION'
2.97001marto@Shemp:~$ 
marto@Shemp:~$ PERL_JSON_BACKEND=JSON::PP perl -MData::Dump=dump -MJSO
+N -E 'my $json = JSON->new;@foo=$json->incr_parse(q!{"check_result!);
+ $bar=$json->incr_parse(q!" : "ok"}!); dump $bar'
{ check_result => "ok" }

... after a cpanm JSON::PP

marto@Shemp:~$ perl -MJSON::PP -e 'print $JSON::PP::VERSION'
4.00
marto@ShempPERL_JSON_BACKEND=JSON::PP perl -MData::Dump=dump -MJSON -E
+ 'my $json = JSON->new;@foo=$json->incr_parse(q!{"check_result!); $ba
+r=$json->incr_parse(q!" : "ok"}!); dump $bar'
{ check_result => "ok" }
marto@Shemp:~$ perl -v

This is perl 5, version 26, subversion 1 (v5.26.1) built for i686-linu
+x-gnu-thread-multi-64int
[download]