Yes, that is correct. The ping utility uses ICMP echo request/ICMP echo reply while, by default, Net::Ping tries to establish a TCP session with the echo port of the target host.
Try making this change to your program:
$p = Net::Ping->new('icmp');
w01b0rji.cube.fred.ch `ping` OK : w01b0rji.cube.fred.ch is alive
w01b0rji.cube.fred.ch $p->ping NOK
svmoloch.flur.fred.ch `ping` OK : svmoloch.flur.fred.ch is alive
svmoloch.flur.fred.ch $p->ping NOK
184.108.40.206 `ping` OK : 220.127.116.11 is alive
18.104.22.168 $p->ping NOK
Just to clarify, what you are seeing here is most likely the result of the specified hosts allowing ICMP echo requests, but not having a service listening to the echo port.