I am not at all sure that STDIN can even be opened in binary mode at all!

Yes it works the same on STDIN as other handles. Specifically, it disables crlf→lf conversion on Windows machines, it stops treating chr(26) as the end of file on non-PerlIO Windows builds, and does nothing elsewhere.

I mean CTL-C, CTL-Z mean things to STDIN although these are certainly valid binary values.

No they don't. They may mean something to the tty/console, but STDIN doesn't even know about the Ctrl key. It doesn't treat character 3 or 26 specially.

>perl -e"print qq{\x03\x1A}" | perl -le"print uc unpack 'H*', <STDIN>"
031A

$perl -e'print qq{\x03\x1A}' | perl -le'print uc unpack "H*", <STDIN>'
031A
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If you want to read binary data, open a file in binmode... local $/ = undef; is not needed.

Not true at all. $/ is quite useful on binary files.

my @records = map parse_rec($_),
              map /(.{$RECSIZE})/sg,
              do { local $/; <$fh> };
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and

my @records;
local $/ = \$RECSIZE;
local *_;
while (<$fh>) {
   push @records, parse_rec($_);
}
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are equivalent to

my @records;
local *_;
while (read($fh, $_, $RECSIZE)) {
   push @records, parse_rec($rec);
}
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Mind you, read is unaffected by $/, but that has nothing to do with whether the file is binary or not.