This works in Perl 5.12.3 (Slackware Linux 13.37 64 bit):

#!/usr/bin/perl

use strict;
use warnings;

my $text="Hello World\n123\njust a short text\n";
my $r=\$text;

open my $f,'<',$r or die "open failed: $!";
print while <$f>;
close $f;
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Output:

Hello World
123
just a short text
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Adding a layer causes a file not found error:

#!/usr/bin/perl

use strict;
use warnings;

my $text="Hello World\n123\njust a short text\n";
my $r=\$text;

open my $f,'<:raw',$r or die "open failed: $!";
print while <$f>;
close $f;
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Output:

open failed: No such file or directory at test.pl line 9.
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The same code worked flawlessly on Perl 5.10.0 (Slackware Linux 13.0 32 bit).

Bug in 5.12.3? Or did I miss some documented change?

perl -V:

Summary of my perl5 (revision 5 version 12 subversion 3) configuration
+:

  Platform:
    osname=linux, osvers=2.6.35.10, archname=x86_64-linux-thread-multi
    uname='linux midas64 2.6.35.10 #2 smp thu jan 6 19:06:19 cst 2011 
+x86_64 amd athlon(tm) ii x2 235e processor authenticamd gnulinux '
    config_args='-de -Dprefix=/usr -Dvendorprefix=/usr -Dcccdlflags=-f
+PIC -Dinstallprefix=/usr -Dlibpth=/usr/local/lib64 /usr/lib64 /lib64 
+-Doptimize=-O2 -fPIC -Dusethreads -Duseithreads -Dpager=/usr/bin/less
+ -isr -Dinc_version_list=5.12.2 5.12.1 5.12.0 5.10.1 5.10.0 5.8.8 5.8
+.7 5.8.6 5.8.5 5.8.4 5.8.3 5.8.2 5.8.1 5.8.0 -Darchname=x86_64-linux'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=und
+ef
    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 -fPIC',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -f
+stack-protector -I/usr/local/include'
    ccversion='', gccversion='4.5.2', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=1
+6
    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'
    libpth=/usr/local/lib64 /usr/lib64 /lib64
    libs=-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lpthread -lc
    perllibs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
    libc=/lib64/libc-2.12.2.so, so=so, useshrplib=false, libperl=libpe
+rl.a
    gnulibc_version='2.12.2'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -fPIC -fstack-protector
+'


Characteristics of this binary (from libperl):
  Compile-time options: MULTIPLICITY PERL_DONT_CREATE_GVSV
                        PERL_IMPLICIT_CONTEXT PERL_MALLOC_WRAP USE_64_
+BIT_ALL
                        USE_64_BIT_INT USE_ITHREADS USE_LARGE_FILES
                        USE_PERLIO USE_PERL_ATOF USE_REENTRANT_API
  Built under linux
  Compiled at Jan 26 2011 12:39:46
  @INC:
    /usr/lib64/perl5/site_perl/5.12.3/x86_64-linux-thread-multi
    /usr/lib64/perl5/site_perl/5.12.3
    /usr/lib64/perl5/vendor_perl/5.12.3/x86_64-linux-thread-multi
    /usr/lib64/perl5/vendor_perl/5.12.3
    /usr/lib64/perl5/5.12.3/x86_64-linux-thread-multi
    /usr/lib64/perl5/5.12.3
    /usr/lib64/perl5/site_perl
    /usr/lib64/perl5/vendor_perl
    .
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Alexander