#!/usr/local/bin/perl -W
$i = 2;
$j = 10;
$k = $i++ - $i++;
print $k,"\n";
$i = 2;
$j = 10;
$k = ++$i - ++$i;
print $k,"\n";
$i = 2;
$j = 10;
$k = $i++ + $i++;
print $k,"\n";
$i = 2;
$j = 10;
$k = ++$i + ++$i;
print $k,"\n";

which outputs:
-1
0
5
8