After my previous reply, I found the time and facilities and ran a quick test and found two mistakes in your suggestion. First, it is eof() (with parens) not eof without parens that triggers ARGV magic.

Second, as I suspected, eof() does read ahead:

 % perl -del existingFile.txt
  DB<1> x eof()
0  ''

  DB<2> x sysseek(*ARGV,0,1)
0  4096

  DB<3>
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So, if on a platform where binmode mattered, binmode() would not have been in effect for the first 4096 (resulting) bytes of the file. (But note that you don't need such a platform to perform this test.) Part of the problem here is that the impact of binmode is handled by the underlying I/O layers, not by Perl, so the bytes pre-read have already passed through the layer where binmode would be applied (but wasn't).

If there is some way to get eof without parens to trigger an ARGV-magic implicit open, then I couldn't figure it out and so didn't test it. So please describe it in more detail.

I'll believe that when I see test code to demonstrate it. My recollection is that eof reads ahead to see whether you are at EOF rather than just returning whether Perl already knew that you were at EOF.

I can see eof allowing code to run just before an implicit close / open, but I'd be surprised to find eof not causing an implicit read() after it caused an implicit open().

Sorry, I'm not in a position to test it myself at the moment.