my $r = '3 + 5 * 2 - 3 * 4';

sub summe {
        my ($left, $right) = @_;        
        my $left_num = evaluate($left);
        my $right_num = evaluate($right);
        return $left_num + $right_num;
}

sub differenz {
        my ($left, $right) = @_;        
        my $left_num = evaluate($left);
        my $right_num = evaluate($right);
        return $left_num - $right_num;
}

sub produkt {
        my ($left, $right) = @_;        
        my $left_num = evaluate($left);
        my $right_num = evaluate($right);
        return $left_num * $right_num;
}

sub quotient {
        my ($left, $right) = @_;        
        my $left_num = evaluate($left);
        my $right_num = evaluate($right);
        return $left_num / $right_num;
}

sub evaluate {
        my $str = shift;
        $str =~ s/^\s*(.*?)\s*$/$1/;
        
        say $str;
        
        if ($str =~ /^\d+$/) {
                return $str;
        }
        
        my ($left, $typ, $right);
        ($left, $typ, $right) = split(/([\-\+])/, $str, 2);
        if (not defined $typ) {
                ($left, $typ, $right) = split(/([\*\/])/, $str, 2);
        }

        if (defined $typ) {
                if ($typ eq '+') {
                        return summe($left, $right);
                }
                elsif ($typ eq '-') {
                        return differenz($left, $right);
                }
                elsif ($typ eq '*') {
                        return produkt($left, $right);
                }
                elsif ($typ eq '/') {
                        return quotient($left, $right);
                }
        }
        else {
                die '?????????';
        }
}

say "Ergebnis: ".evaluate($r);