From @Leont

This is a bug report for perl from fawaka@​gmail.com,
generated with the help of perlbug 1.39 running under perl 5.16.1.

The XS Typemap file FILE* pushes a :stdio layer on top of the
filehandle but never removes it, carpeting over the existing layers.
This is particularly troublesome with encoding layers. For example
(this uses Sys​::Mmap, but any module using the FILE* typemap should
give the same result)​:

$ perl -MSys​::Mmap -E 'open my $fh, "<​:encoding(utf-8)", "Changes";
say join "​:", PerlIO​::get_layers($fh); mmap my $var, -s $fh,
PROT__READ, MAP_SHARED, $fh, 0; say join "​:", PerlIO​::get_layers($fh)'
unix​:perlio​:encoding(utf-8-strict)​:utf8
unix​:perlio​:encoding(utf-8-strict)​:utf8​:stdio

The latter is effectively the same as just «stdio» (until a
PerlIO_releaseFILE/PerlIO_pop). A conversion should never result in a
variable changing in nature or lose data, this is plain wrong.

Flags​:
  category=core
  severity=low

Site configuration information for perl 5.16.1​:

Configured by leon at Thu Aug 9 12​:30​:13 EEST 2012.

Summary of my perl5 (revision 5 version 16 subversion 1) configuration​:

  Platform​:
  osname=linux, osvers=3.2.0-27-generic, archname=x86_64-linux-thread-multi
  uname='linux leon-laptop 3.2.0-27-generic #43-ubuntu smp fri jul 6
14​:25​:57 utc 2012 x86_64 x86_64 x86_64 gnulinux '
  config_args='-de
-Dprefix=/home/leon/perl5/perlbrew/perls/perl-5.16.1 -Duseshrplib
-Dusethreads'
  hint=recommended, useposix=true, d_sigaction=define
  useithreads=define, usemultiplicity=define
  useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
  use64bitint=define, use64bitall=define, uselongdouble=undef
  usemymalloc=n, bincompat5005=undef
  Compiler​:
  cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing
-pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_BITS=64',
  optimize='-O2',
  cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe
-fstack-protector -I/usr/local/include'
  ccversion='', gccversion='4.6.3', gccosandvers=''
  intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
  d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
  ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t',
lseeksize=8
  alignbytes=8, prototype=define
  Linker and Libraries​:
  ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
  libpth=/usr/local/lib /lib/x86_64-linux-gnu /lib/../lib
/usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib /usr/lib
  libs=-lnsl -ldb -ldl -lm -lcrypt -lutil -lpthread -lc
  perllibs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
  libc=, so=so, useshrplib=true, libperl=libperl.so
  gnulibc_version='2.15'
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E
-Wl,-rpath,/home/leon/perl5/perlbrew/perls/perl-5.16.1/lib/5.16.1/x86_64-linux-thread-multi/CORE'
  cccdlflags='-fPIC', lddlflags='-shared -O2 -L/usr/local/lib
-fstack-protector'

Locally applied patches​:

@​INC for perl 5.16.1​:
  /home/leon/perl5/perlbrew/perls/perl-5.16.1/lib/site_perl/5.16.1/x86_64-linux-thread-multi
  /home/leon/perl5/perlbrew/perls/perl-5.16.1/lib/site_perl/5.16.1
  /home/leon/perl5/perlbrew/perls/perl-5.16.1/lib/5.16.1/x86_64-linux-thread-multi
  /home/leon/perl5/perlbrew/perls/perl-5.16.1/lib/5.16.1
  .

Environment for perl 5.16.1​:
  HOME=/home/leon
  LANG=en_US.UTF-8
  LANGUAGE=en_US​:en
  LC_ADDRESS=en_US.UTF-8
  LC_IDENTIFICATION=en_US.UTF-8
  LC_MEASUREMENT=en_US.UTF-8
  LC_MONETARY=en_US.UTF-8
  LC_NAME=en_US.UTF-8
  LC_NUMERIC=en_US.UTF-8
  LC_PAPER=en_US.UTF-8
  LC_TELEPHONE=en_US.UTF-8
  LC_TIME=en_US.UTF-8
  LD_LIBRARY_PATH (unset)
  LOGDIR (unset)
  PATH=/home/leon/perl5/perlbrew/bin​:/home/leon/perl5/perlbrew/perls/perl-5.16.1/bin​:/home/leon/bin​:/usr/lib/lightdm/lightdm​:/usr/local/sbin​:/usr/local/bin​:/usr/sbin​:/usr/bin​:/sbin​:/bin​:/usr/games
  PERLBREW_HOME=/home/leon/.perlbrew
  PERLBREW_PATH=/home/leon/perl5/perlbrew/bin​:/home/leon/perl5/perlbrew/perls/perl-5.16.1/bin
  PERLBREW_PERL=perl-5.16.1
  PERLBREW_ROOT=/home/leon/perl5/perlbrew
  PERLBREW_VERSION=0.25
  PERL_BADLANG (unset)
  SHELL=/bin/bash