As of Perl 5.23, using :utf8 binmode on filehandles opened with sysopen causes a warning (or at least it does as soon as someone tries to syswrite/sysread).

From what I read in perldoc perlport -- it's a portability issue because sysseek'ing and tell'ing get garbled and confused. Makes perfect sense why you'd want to avoid it.

I've written the following statement as part of a documentation effort, based on my understanding of the new deprecation, and I wonder if any of it is inaccurate or even plain less-than-ideal. I have asked the same question to the perl5 core hackers on IRC, but while I'm waiting on a response from them, I'm still interested in what you think about it.

Specifically because I'm not a C expert or perl-guts guru, I want to be doubly sure this is right before I start disseminating it:

As a rule of thumb, and to avoid confusion, stick to the use of utf8 binmode for unicode text streams only, or if you don't have a compelling need for that, you can use lower level system IO via sysopen/sysread/syswrite and be done with it. In the end, bytes at rest in files are just bytes. If you aren't trying to encode your text as UTF-8 strict, you can safely use system IO without adding any additional IO disciplines/layers and you should be fine. With system IO, no translations will occur on your input/output streams that could cause your file content to be mangled. You can write raw text or even binary streams and never worry.

UPDATE: If you're reading this on the front page, please click on in-- the discussion on this post is a worthwhile read, and contains corrections.