:) but what is it supposed to do? I add

print join ' ', 1, PerlIO::get_layers($fh), "\n";
binmode($fh, ':raw:perlio') or warn "UHOH $! \n$^E \n$@\n ";
print join ' ', 2, PerlIO::get_layers($fh), "\n";
binmode($fh, ':raw') or warn "UHOH $! \n$^E \n$@\n ";
print join ' ', 3, PerlIO::get_layers($fh), "\n";
binmode($fh, ':raw:raw:raw:raw') or warn "UHOH $! \n$^E \n$@\n ";
print join ' ', 4, PerlIO::get_layers($fh), "\n";
binmode($fh, ':raw:raw:raw:raw:unix:crlf') or warn "UHOH $! \n$^E \n$@
+\n ";
print join ' ', 5, PerlIO::get_layers($fh), "\n";
for(1..3){
    binmode($fh, ':raw:pop') or warn "UHOH $! \n$^E \n$@\n ";
    print join ' ', 6, PerlIO::get_layers($fh), "\n";
}
#~ binmode($fh, ':raw:crlf:perlio') or warn "UHOH $! \n$^E \n$@\n ";
binmode($fh, ':pop:pop:win32') or warn "UHOH $! \n$^E \n$@\n ";
print join ' ', 6, PerlIO::get_layers($fh), "\n";
[download]

and I get

1 unix crlf
2 unix crlf perlio
3 unix crlf perlio
4 unix crlf perlio
5 unix crlf perlio unix crlf
6 unix crlf perlio unix
6 unix crlf perlio
6 unix crlf
6 win32
[download]

in addition to the file-already-open

if turning off unix and turning it on again, or turning it on twice, is wrong, perl should warn or die

PerlIO seems thin, OTOH, the test suite lists a TODO #56644: PerlIO resource leaks on open() and then :pop in :unix and :stdio but its closed

Any way you look at it there is nonsense around :)