I wonder how "/^" could match anything in the command you've given. Anyway, df /foo/bar lists the data only for the filesystem that contains the file /foo/bar (which can be the root file system, or a fs mounted on /foo or on /foo/bar).

The pure perl solution is to use ustat or statfs:

sub __NR_statfs () { 99 }; $file = "/"; $s = pack "x256"; -1 == syscal
+l(__NR_statfs, "".$file, $s) and die "error statfs: $!"; (undef, $bsi
+ze, undef, $bfree, $bavail) = unpack "l!5", $s; print $bavail*$bsize/
+1024, " kilobytes available\n";
[download]

Update: statfs is linux-specific (Update: except not really, but the structure might be different, I don't know). Ustat might be usable on other systems too.

Update 2006 sep: note that system call numbers are different on every architecture (OS and CPU type). You have to look up the syscall number of your architecture.