elixir, 220 bytes

Result

AC

Duration

1962ms

Code [DL]

defmodule F do
def f(s,t) do
case IO.gets("") do
:eof->IO.write String.slice(String.trim_trailing(t)<>String.duplicate("*",99),0,String.length String.trim_trailing s)
t->IO.write s
f(s,t)
end
end
end
F.f(IO.gets(""),"")

stdin

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                 K                                

stdout

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                 K***************

stderr

strace