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');
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Update

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

160.69.125.142 `ping` OK : 160.69.125.142 is alive
160.69.125.142 $p->ping NOK
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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.