$var = '09';
$trans = '/3+5';

eval "$var$trans";  # eval gets '09/3+5'
                    # Illegal octal digit '9'

eval $var.$trans;   # eval gets '09/3+5'
                    # Illegal octal digit '9'

eval '$var'.$trans; # eval gets '$var/3+5'
                    # result: 8