Mostly because I am sure that I won't be getting any interference from IOLayers, Unicode conversions or whatever. That may be paranoia, but I believe that I have had the situation where a random piece of binary data has looked sufficiently like unicode to cause is to be upgraded by some action.

While indeed there are situations in which it can be necessary to use sys*(), it is also true that binmode(), or open()'s '<:raw' mode should that care of your concernes altogether.

Then one can use Perl's typical IO {operators,functions}. Since the OP underlined that he has to process a whole 9Gb file, chances are that it may be possible to do it one chunk (whatever this may mean, size-wise) at a time with good'ol while (<$fh>), provided that $/ is set accordingly (e.g. local $/=\512).

This may have been on 5.6.1 before the unicode support was sorted out--by why risk it?

AFAIKnew unicode support has not been "sorted out", nay, notwithstanding the fact that I do not need it nor have I ever used it, it is my understanding that it's being constantly improved. What has been sorted out is unicode automatic handling (depending on an environment variable, which somehow forced *NIX users to use binmode() too, something they're not used to!)