What device are you running this on?

Here it works fine (18F57Q43)
Code:
Start:
    HSEROUT["Start", 10,13]

    arraywrite topline, ["ABCabc"]
    FOR X=0 TO 6
        HSEROUT [DEC topline[x],10,13]
    next
Outputs:
Code:
Start
65
66
67
97
98
99
23
With the 23 at the end being because you're reading past the last character written to the array.