So, what I learned:

• \*ARGV and ${^LAST_FH} do refer to the same file
• \*ARGVOUT and my $sh = select; do refer to the same file
• Almost sufficient: perl -pi.orig -e "binmode ARGV; binmode ARGVOUT; s/w/v/g" src.bin : seemed to confirm that source file and output file were same length.
• I couldn't get perl -pi.orig -Mopen=IN,:bytes,OUT,:bytes -e "s/w/v/g" src.bin to work the same as the dual bin-mode (I tried with :raw vs :bytes, and only the IN or OUT or both...
• Unfortunately, even with the binmode, I found it wasn't sufficient: changing the input file so the first null-separated record (aka "line") has all the various EOL-like sequences showed me wrong: perl -e "binmode STDOUT; $,=qq(\0); print qq(n\nr\rnr\n\rrn\r\nend), qw(encoded file), qq(r:\r\0n:\n\0nr:\n\r\0rn:\r\n), qw(with EOL-like sequences)" > src.bin ⇒ perl -pi pi.pl src.bin & dir src.bin* ⇒ the output is one byte shorter than the input, because the binmode(ARGV) is too late for the first read (as was mentioned in the binmode and one-liners). Also, I'd lost the $/=qq(\0) at some point in the BEGIN block; when I re-inserted that, I found it wasn't working (because the first record had only been reading to the first newline, instead of the first null, and thus masking the fact that binmode wasn't applied for the first read).
• I tried doing some tricks with eof (with or without empty parentheses), to no avail.
• I tried doing BEGIN { ...; binmode ARGV; }, but it didn't help

At this point, I'm just going to give in do it without the magic -pi. It frustrates me that I cannot get this, but since I have a less magic way working, I guess I'll have to live with it.

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