#AHK , 116 bytes

Loop,Read,%1%
{a=%A_LoopReadLine%`n
Loop,Parse,a
{Send % c:=A_LoopField
If((f:=c SubStr(f,1,4))=="olleh")
ExitApp
}}

There's nothing clever or magical in there, really. The variable %1% is the first passed argument and should be a file path with the stream. The file must be saved as it is updated but the code will read to the end even if it expands after the reading starts.

AHK , 116 bytes

Loop,Read,%1%
{a=%A_LoopReadLine%`n
Loop,Parse,a
{Send % c:=A_LoopField
If((f:=c SubStr(f,1,4))=="olleh")
ExitApp
}}

There's nothing clever or magical in there, really. The variable %1% is the first passed argument and should be a file path with the stream. The file must be saved as it is updated but the code will read to the end even if it expands after the reading starts.