Few days ago there was some important perl update:
Thu Sep 5 22:23:09 2013 >>> dev-lang/perl-5.18.1
And today I noticed that my rrdtool stopped gathering data. I got the idea, that it was perl-related (the script is written in perl ), but didn't get the missing link (I was like „wtf perl? needz to perl-cleaner --all?”, but it didn't work out). But then, in a flashback, there it was, some @preserved-rebuild… so I tried a simple emerge -1 rrdtool and it's working again.
Would be nice, with all those slots, auto-rebuilds and other magic to do the trick and rebuild rrdtool as well. ;)
That is what perl-cleaner is for. You should run it after you update dev-lang/perl.
*** This bug has been marked as a duplicate of bug 41124 ***