Hello, suppose I have a huge file "test_file.csv" (about 40GB).
...
But instead of prepending the line, it is replacing the existing lines!!

I suspect most monks reading this would be entirely unsurprised that the data were overwritten rather than prepending. That's because experience has taught us this (not just in Perl, but in using pretty much any language to perform file manipulation). So forgive me if I'm wrong in assuming that you are relatively new to all this.

If you are new to it then perhaps you could be so good as to justify why you have a CSV file that's 40GB in size? Experience says that this is very large for a CSV file. If you could reduce this size by an order of magnitude or more then the problem effectively vanishes. Of course it is entirely possible that every one of those 40 billion bytes is absolutely essential, just rather unlikely.