You are right, "localhost" does not resolve:

% perl -MSocket -e'defined inet_aton("localhost") or die;'
Died at -e line 1.
[download]

I forgot that I meddled with /etc/hosts trying to resolve a different issue, and I guess I didn't restore it

Now that I have restored it, the error is gone.
Strange that I did not get this problem until now, as it has been days since this meddling...