No, I meant that I did

$ sudo emerge net-analyzer/rrdtool
$ sudo perl -MCPAN -e shell
[download]

in one case, and everything went like a charm, and

$ sudo apt-get install rrdtool
$ sudo perl -MCPAN -e shell
[download]

in the other one, and I couldn't get it to work.

Of course I'm also trying to install it manually, but as tphyahoo pointed out at Re: Seeking advice re installing RRDTool::OO, I guess that in fact the problem is due to the absence of the RRDs module, although it should be there...