Filesystem           1K-blocks    Used Available Use% Mounted on
/dev/mapper/vg_mail-lv_var
                      94959436 1415576  88713448   2% /var
[download]

That explains why the scalars are not set for you. If your Filesystem paths are so long as to overflow like this you will need to use -P. That will avoid the wrapping and hence the assignment errors. Note that this is a demonstrably good reason not to shell out but to use a module instead.

Here is an SSCCE demonstrating this use of -P:

#!/usr/bin/env perl 
use strict;
use warnings;

my @df = `df -P /var`;
my ($used, $free) = (split (/\s+/, $df[1]))[2,3];
print "used $used, free $free\n";
[download]

The split acts on $df[1] (which is the second line returned by df) and splits it on whitespace (like awk does). You then want only the 3rd and 4th fields for your used and free figures but indexing in Perl starts at zero hence the slice of [2,3]. Hopefully that makes it clear enough?