Re context, I beg to differ. In my original post, I stated that the only change was to line one, and both working and non working versions where being called in the same environment with the same command line. Further, I had myself determined that I was entering a while loop unexpectedly, and that was causing my issue. The rest of the script was not relevant. However, when you start to dig deeper, and you get further information such as the link to the IO perldoc, you start to look elsewhere and that link made me realise that maybe - and it is just maybe - the command line arguments were having an impact. Having said that, they are the same command line arguments for both working and non working versions, so something which remains unidentified has changed in the core Perl processing.