Ruby -- issues with unary + used on strings

ruby_parser_problem.rb

$ ./irb 
>> def ret_str
>> "hello "
>> end
=> nil
>> @ins_var = "world"
=> "world"
>> ret_str +@ins_var
NoMethodError: undefined method `+@' for "world":String
	from (irb):5
	from ./irb:12:in `<main>'
>> ins_var
NameError: undefined local variable or method `ins_var' for main:Object
	from (irb):6
	from ./irb:12:in `<main>'
>> self.ins_var
NoMethodError: undefined method `ins_var' for main:Object
	from (irb):7
	from ./irb:12:in `<main>'
>> "hello " +@ins_var
=> "hello world"
>> str = "hello "
=> "hello "
>> str +@ins_var
=> "hello world"
>> ret_str+@ins_var
=> "hello world"
>> str = "world"
=> "world"
>> ret_str +str
NoMethodError: undefined method `+@' for "world":String