perl can't find the module in @INC !
You must install Filesys::Df module from cpan.org with cpan shell or download the source and install it.
or
include for example with use lib 'path_to_your_pm_lib';
if you don't have the cpan shell :
Enter from the shell command line
perl -MCPAN -e 'install Filesys::Df'
HTH,
PooLPi