If it's a homework than maybe eval can't be used yet.

You just need to read in the operator (- *) just like you read in the Name. Then you can use if.

if ($operator eq '-') {
  $c = $a - $b;
} elsif ($operator eq '*') {
  # * code here
} else {
 $c = 'invalid operator';
}
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Read about the if statement and comparison operatiors (eq == ne !=).