I don't think split function is an issue.

But you did, see split function.

it only read the line up to length = 51 and then it will continue to read on the nextline. I already checked the file, there is no newline in the file.

There is a saying about that #11943 Ah yes, and you are the first person to have noticed this bug since 1987. Sure.

It is not impossible to discover bugs in perl, I've seen it many times, but each time, it was by someone who provided a Short, Self Contained, Correct (Compilable), Example

We only have your word that there is no newline in your datafile, or that a problem exists at all. We need code that proves it :) see How do I post a question effectively?