snobol, 98 bytes

Result

AC

Duration

707ms

Code [DL]

	T = TRIM(INPUT)
F	LGT(OUTPUT = T,K = TRIM(INPUT))	:S(F)
	OUTPUT = K DUPL(1,SIZE(T) - SIZE(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
                                        T
                                        T
                                        T
                                        T
                                        T
                                        T
                                        T
          K111111111111111111111111111111

stderr

strace

10    execve("/bin/sh", ["/bin/sh", "/root/script", "/volume/CODE"], 0x7ffe86b36110 /* 9 vars */) = 0
11    execve("/bin/cat", ["cat", "-"], 0x55939f617d38 /* 9 vars */) = 0
12    execve("/usr/local/bin/snobol4", ["/usr/local/bin/snobol4", "/volume/CODE"], 0x55939f617d50 /* 9 vars */ <unfinished ...>
11    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=11, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
12    <... execve resumed> )            = 0
12    +++ exited with 0 +++
10    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---
10    +++ exited with 0 +++