[perl #35180] inconsistency between PerlIO::get_layers and open/binmode

> > > 
> > > open F, "foo.pl";
> > > 
> > > 
> > > # now try to open a temp file with same layers as F
> > > my $tmp = undef;
> > > my $F_layers = join "", map {":$_"} PerlIO::get_layers(F); 
> > > open $tmp, "+>$F_layers", undef;
> > > 
> > > my $tmp_layers = join "", map {":$_"} PerlIO::get_layers($tmp); 
> > > 
> > > print "MISMATCH !! $F_layers / $tmp_layers\n" unless $F_layers eq
> > > $tmp_layers;
> > > 
> > > # on Win32 : MISMATCH !! :unix:crlf / :unix:crlf:unix
> > 
> > I'm not sure that's really a bug, since open pushes some layers by
> > defaults depending on the platform. That's more a minor annoyance.
> 
> ":unix:crlf" seem to be the default layers on this platform.
> When $tmp is opened
> 
>     open $tmp, "+>$F_layers", undef;
> 
> ":unix:crlf" ($F_layers) are pushed on top of the default layers
> resulting in ":unix:crlf:unix:crlf" for $tmp. However ":crlf" is
> documented to be pushed only once
> 
>     :crlf
>     A layer that implements DOS/Windows like CRLF line endings.
>     ... ...
>     Note that this layer likes to be one of its kind: it silently
>     ignores attempts to be pushed into the layer stack more than once.
>     ... ...
> 
> and the result is ":unix:crlf:unix" (the rightmost ":crlf" is silently
> ignored). 
> 
> IMHO your code behaves exactly as documented.
> 
But the odd thing is if undef is replaced by a file name it works 
as original poster expected. 
So it seems the magic undef for temp file code is getting extra 
set of layers.

• [perl #35180] inconsistency between PerlIO::get_layers and open/binmode by Nick Ing-Simmons via RT