Просто удалите /a
set "hlo=Hello"
set "hlo=%hlo% World"
Потому что set /a
для выполнения арифметики, а не для строковых операций
set /?
...
Two new switches have been added to the SET command:
SET /A expression
SET /P variable=[promptString]
The /A switch specifies that the string to the right of the equal sign
is a numerical expression that is evaluated. The expression evaluator
is pretty simple and supports the following operations, in decreasing
order of precedence:
...