% cat /proc/swaps
Filename                        Type            Size    Used    Priority
/dev/ide/host0/bus0/target1/lun0/part3 partition        2056312 0       -1

##</code><code>##

open IN, "</proc/swaps" or die "No swap file: $!\n";
<IN>;
my @fields = split <IN>;
$field[3] = 0 unless $field[3];
print "swap used = $field[3]\n";