snobol, 143 bytes

Result

AC

Duration

686ms

Code [DL]

	T = OUTPUT = INPUT
	S = INPUT
F	OUTPUT = T
	DIFFER(S,K = INPUT)	:S(S)F(F)
S
H	EQ(SIZE(K = TRIM(K) 1),SIZE(TRIM(T)))	:S(T)F(H)
T	OUTPUT = K
END

stdin

                                               T  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                     K            

stdout

                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                               T  
                                     K1111111111

stderr

strace