add this:

print $greeting;
chomp($operator = <STDIN>);
#your other prints and stuff here
$c = $a - $b if $operator eq "-";
$c = $a * $b if $operator eq "*";
[download]

The if statement could also be written as such:

if ($operator eq "-")
{
  $c = $a - $b;
} else {
  $c = $a * $b;
}
[download]