open($f, "<:encoding(foo)", "infoo"); open($f, ">:encoding(bar)", "outbar");
use Encode qw(:fallbacks); $PerlIO::encoding::fallback = FB_PERLQQ;
On input, convert the bytes expected to be in the specified character set and encoding to Perl string data (Unicode and Perl's internal Unicode encoding, UTF-8). On output, convert Perl string data into the specified character set and encoding.
When the layer is pushed the current value of $PerlIO::encoding::fallback is saved and used as the check argument when calling the Encodings encode and decode.