$ perl -Mstrict -Mwarnings -le '
    {
        my %op_func = (
            "+" => sub { $_[0] + $_[1] },
            "-" => sub { $_[0] - $_[1] },
            "*" => sub { $_[0] * $_[1] },
            "/" => sub { $_[0] / $_[1] },
        );

        sub expr {
            my ($lhs, $op, $rhs) = split " ", shift;
            $op_func{$op}->($lhs, $rhs);
        }
    }

    my @ops = ("+","-","*","/");

    foreach my $op (@ops){
        print "4 $op 5 = ", expr("4 $op 5");
    } 
'
4 + 5 = 9
4 - 5 = -1
4 * 5 = 20
4 / 5 = 0.8