$ 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