sub myopen {
   my (..., $perlio, ...) = @_;
   $perlio ||= '';

   ...
   open(my $fh, ">$perlio", ...) or die ...;
   ...
}

myopen(..., ':raw:perlio', ...);  # bin mode with buffering
myopen(..., '',            ...);  # default
myopen(...,                   );  # default