I can confirm this:

$ perl --version

This is perl, v5.8.8 built for i486-linux-gnu-thread-multi
...

$ perl -MPerlIO -e 'open($fh, "<", "/dev/null");
print join(", ", PerlIO::get_layers($fh)), "\n";'
unix, perlio
$ perl -MPerlIO -e 'open($fh, "<:raw", "/dev/null");
print join(", ", PerlIO::get_layers($fh)), "\n";'
unix
$ perl -MPerlIO -e 'open($fh, "<", "/dev/null");
binmode $fh;
print join(", ", PerlIO::get_layers($fh)), "\n";'
unix, perlio
[download]

Looks like a bug to me, since the behaviour appears to contradict the description in the docs (either a doc bug or a code bug).

For what it's worth, I tend to use the binmode $fh; variant, but that's a habit formed in pre-layer times.