I *think* the problem is to do with you having multiple RRDTool Perl libraries installed on your system.
The mrtg-pnsclient script is working correctly, since it runs independently. The problem is where MRTG (or RRD?) is calling the Float routines.
Check to see how many RRDs.pm you have installed on your system. If you can, run something like 'find / -name RRDs.pm -print' to see how many of them there are. If you find more than one, make sure that they are all the same version! It may be you have one version in the rrdtool directory, and another in /usr/lib/perl.
Are things working with other external program calls?
We are using an older version of Float here, 1.44, but it looks like the problem is not in the Float libraries themselves, but in how they are being called (by MRTG, or RRDs.pm).
UNIX Systems, ITSS, University of Auckland, NZ
Woe unto them that rise up early in the morning... -- Isaiah 5:11