$ LANG=C; df -a
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda6              3842376    873884   2773304  24% /
tmpfs                  1557800         0   1557800   0% /lib/init/rw
proc                         0         0         0   -  /proc
...
/dev/mapper/vol1-logdeb1
                       5160576   4457740    440692  92% /var
[download]

Hmm dunno what your ubuntu server does but my debian system shows "/" correct. Dont think this is standard ...

Perl uses system tools to determine the space i think so if df is wrong Perl is too? Checked the debian Servers too they do ..:

Dateisystem          1K-Blöcke   Benutzt Verfügbar Ben% Eingehängt auf
/dev/hda2             75981028  17982544  54138868  25% /
tmpfs                   513980         0    513980   0% /lib/init/rw
proc                         0         0         0   -  /proc
...
[download]

Btw: I wouldnt use a graphical interface for a server.
It is another (possible) security hole not talking about the waste of ressources (if it is your home server without internet access it is ok ...)