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";
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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.

df -P | awk '/^\//{printf("%5s %s\n", $5, $6);}'
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df -P | perl -ne 'printf("%5s %s\n", (split(/\s+/, $_))[4, 5])'
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use Filesys::Statvfs;

open MOUNTED, "</proc/mounts" || die "Could not open /proc/mounted\n";

while (my $mountpoint = <MOUNTED>) {
    my ($bsize, $blocks, $bfree, $bavail, $files, $ffree, $namelen);
    my $path = (split(/\s+/, $mountpoint))[1];
if (1) {
    ($ftype, $bsize, $blocks, $bfree, $files,
     $ffree, $bavail) = statvfs("/tmp");
} else {  # This is not very portable
    my $buf = "\0"x64;
    syscall(99, $path, $buf) == 0 or die "Bad mountpoint $path\n";

    ($bsize, $blocks, $bfree, $bavail, $files, $ffree, $namelen)
        = unpack  "x4 L6 x8 L", $buf;

}
    next unless $blocks;

    print <<EOT;
                            path: $path
     Optimal transfer block size: $bsize
           Blocks in file system: $blocks
                     Blocks free: $bfree (${\(int $bfree/$blocks*100)}
+%)
           User blocks available: $bavail (${\(int $bavail/$blocks*100
+)}%)
                          Inodes: $files
                     Free inodes: $ffree (${\(int $ffree/$files*100)}%
+)
EOT
}
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df -k | perl -ne '/(\S+\s+){4}(\S+)/&&print$2.$/' -
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Perl's "-a" command-line option allows you to pretend that you're using something that resembles "awk" (that's why it's "-a") -- differences between "perl -a" and "awk" are that perl uses @F where awk uses $1,$2,..., and the first token on the line is $F[0] instead of $1:

 df -k | perl -lane 'print $F[3]'
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(the "-l" option helps make line-feed handling more like awk as well). I presume this still doesn't do exactly what the OP intended, since it prints a line of output for every line of input from "df -k"; but the OP's use of "grep '/^' " seems odd -- not sure what's intended by that.

To limit the output to a particular line, one can cite the target path as an arg to "df", and/or use a condition in the perl script:

 df -k / | perl -lane 'print $F[3] if /\d/' # skips column headings

# or just:

 df -k | perl -lane 'print $F[3] if m{/$}'  # only prints value for "/
+"
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Thanks for the info graff, the use of an expression grep "/^" in the middle of my arguement will only report back the line of output in the df -k that has a slash that is immediately followed by an end of line tag (anchor)- thereby only returning the root partition, as opposed to only doing a basic grep for a slash, which will return every slice starting with a slash (working in ksh).
Is there any way I can incorporate your 'one-liner' into my script? As this is only one function out of several that needs to be performed.
Good judgement comes with experience. Unfortunately, the experience usually comes from bad judgement.