If what you want is to get the available space on a specific disk volume from within a perl script, I'd do something like this:

my $targ_vol = "/";
my @df_output = `df -k $targ_vol`;

# output should be two lines:
# first line is column headings, second line is data

my %df_data;
if ( @df_output ) {
    my @headings = split( /\s+/, $df_output[0] );
    my @data = split( /\s+/, $df_output[1] );
    @df_data{@headings} = @data;
}
print "available on $targ_vol: $df_data{Avail}\n";

# (your version of df might use some string similar to but different f
+rom "Avail")
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You shouldn't have a problem if you're just looking at "/".

(update: if you're looking at some path other than "/", you need to be careful about knowing whether it's on some different file system, and figuring out whether that file system is currently mounted -- if it isn't, df will give you info about "/" instead. And God help you if it's mounted as NFS, and the NFS server has gone down...)

(update: BTW, when I try "df | egrep '/^'", nothing comes out, which is what I would expect; but if I do "df | egrep '/$'", I get the line that ends with a slash. That's what I don't understand about your usage in the OP -- I think it can't work as posted, unless your "egrep" is way different from mine...)