use PerlIO;

my $fd;
open($fd, '+>>:raw', 'somefile.data') || die $!;

my $layers = ':' . join(':', PerlIO::get_layers($fd));

print "$layers\n";
#
# this creates a new file named "&3"
#
my $fn = fileno($fd);
my $fd2;
open($fd2, '+>>' . $layers, '&' . $fn) || die $!;
#
# this dies with "Invalid argument at filemode.pl line ..."
#
my $fd3;
open($fd3, '+>>' . $layers . ' &' . $fn) || die $!;

close $fd;
close $fd2;
close $fd3;