Of course, you don't want to be using a single, hardcoded file name. If someone else submits a comment at the same time, your block.txt will get overwritten, avoiding your scan. Look into File::Temp.

Updated: (repeating messaged clarification for the benefit of others) The problem isn't with the code you've shown. Suppose I click submit. Your code dumps my comment into block.txt. Before the file is read in by your scanner, someone else submits a comment. Now your scanner checks block.txt, but not MY block.txt. Depending on how the rest of your code works, my comment is either dropped (bad) or not scanned (also bad).