So, do you want to be able to test what happens (or detect it) when within these few CPU cycles some other process modifies the file while your Perl process does IO? Or do you want to ignore this risk?

As a side remark, on top of race conditions you also have buffered IO routines. Do these keep IOing even if the file is gone / file permissions changed until the buffer is exhausted? If that's the case you will always get a delayed picture.