>perl -wMstrict -le
"use 5.010;
 ;;
 sub expr {
   state $op_func = {
     '+' => sub { $_[0] + $_[1] },
     '-' => sub { $_[0] - $_[1] },
     '*' => sub { $_[0] * $_[1] },
     '/' => sub { $_[0] / $_[1] },
     };
   ;;
   my ($lhs, $op, $rhs) = split ' ', shift;
   return $op_func->{$op}->($lhs, $rhs);
   }
 ;;
 for my $op (qw(+ - * /)) {
   my $exp = qq{4 $op 5};
   print $exp, ' = ', expr($exp);
   }
"
4 + 5 = 9
4 - 5 = -1
4 * 5 = 20
4 / 5 = 0.8