print ++$i + 1;          # prints 2, increment and than add 1
print $i++ + 1;          # prints 2, $i==1, take the old $i (1) and add 1, the increment happens after the sum was calculated  
print $i + $i++;         # prints 5, $i==2, the increments happens first and we get 2+3
print ++$i + ++$i;       # prints 10, $i==3, the pre increment is done twice because it's precedens on '+' and than we get 5+5
print ++$i + $i++ + 1;   # prints 14, $i==5, didn't figure out yet