$ perl -v
This is perl, v5.8.6 built for darwin-thread-multi-2level
--snip--

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

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

$ 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