From the looks of it you have a couple problems with your system call.

1) Your web daemon is (Hopefully) running as the user nobody, check to make sure that the user nobody has access to run df. If nobody does not have permissions, you will need to build a setuid script in C (unless the setuid/apache/perl 'bug' has been fixed)

2) As it is, you are storing what should be an array in a scaler. Run the command from the shell. I bet you get more than one line back.

3) While in this particular example, you aren't passing anything to your system call, in the future you might consider it. Don't please, at least without all the things that davis meantions. Even then, seriously reconsider passing ANYTHING to the shell. It can be a MAJOR security threat.