my $i = 0;             # init $i and set to 0

print ++$i + 1;        # answer: 1 (0 + 1) post-operation -> $i == 1 now
print $i++ + 1;        # answer: 3 (2 + 1) post-operation -> $i == 2 now
print $i + $i++;       # answer: 5 (2 + 3) post-operation -> $i == 3 now
print ++$i + ++$i;     # answer: 7 (3 + 4) post-operation -> $i == 7 now
print ++$i + $i++ + 1; # answer: 17 (7 + 9 + 1) post-operation -> $i == 17 now