Greetings Appy16,
I am kind of new to perl, so I admit I very well could be wrong about this. From reading the Net::Ping::External page (http://search.cpan.org/~colinm/Net-Ping-External-0.10/External.pm) there is a warning at the bottom. It states

'This module calls whatever "ping" program it first finds in your PATH environment variable.'

I do know that crontabs typically do not have the PATH environment variable set. Yet when you run it by hand, the PATH environment variable is set. So Net::Ping::External can find ping.
As I said, I am new to perl, so I could be totally off. Hopefully this will set you in the right direction.