Filesystem 1K-blocks Used Available Use% Mounted on
/dev/mapper/vg_mail-lv_var
94959436 1415576 88713448 2% /var
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";
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?