$ perl -v
This is perl, v5.8.5 built for i386-linux-thread-multi
--snip--

$ perl -MPerlIO -le 'open IN, "<"            ,"a.txt";             print for PerlIO::get_layers(IN)'
unix
perlio

$ perl -MPerlIO -le 'open IN, "<"            ,"a.txt"; binmode IN; print for PerlIO::get_layers(IN)'
unix
perlio

$ perl -MPerlIO -le 'open IN, "<:raw"        ,"a.txt";             print for PerlIO::get_layers(IN)'
unix

$ perl -MPerlIO -le 'open IN, "<:raw:perlio" ,"a.txt";             print for PerlIO::get_layers(IN)'
unix
perlio