スカラーリファレンスのbless

JPerl Advend Calendarで無名スカラーリファレンスネタをやったんだけど

さくらとかでFreeBSD6.1で

bless \do{""}, "class";

とかやるとModification of a read-only value attemptedとか言って怒られる。
CentOSとかだとだいじょぶなんだけどなー
なんでだ?

$perl -V
Summary of my perl5 (revision 5 version 8 subversion 8) configuration:
  Platform:
    osname=freebsd, osvers=6.1-release-p20, archname=i386-freebsd-64int
    uname='freebsd www900.sakura.ne.jp 6.1-release-p20 freebsd 6.1-release-p20 #0: wed oct 17                                                                                                                                               10:22:04 jst 2007 root@www900.sakura.ne.jp:usrsrcsysi386compilesakura9 i386 '
    config_args='-sde -Dprefix=/usr/local -Darchlib=/usr/local/lib/perl5/5.8.8/mach -Dprivlib=                                                                                                                                              /usr/local/lib/perl5/5.8.8 -Dman3dir=/usr/local/lib/perl5/5.8.8/perl/man/man3 -Dman1dir=/usr/l                                                                                                                                              ocal/man/man1 -Dsitearch=/usr/local/lib/perl5/site_perl/5.8.8/mach -Dsitelib=/usr/local/lib/pe                                                                                                                                              rl5/site_perl/5.8.8 -Dscriptdir=/usr/local/bin -Dsiteman3dir=/usr/local/lib/perl5/5.8.8/man/ma                                                                                                                                              n3 -Dsiteman1dir=/usr/local/man/man1 -Ui_malloc -Ui_iconv -Uinstallusrbinperl -Dcc=cc -Duseshr                                                                                                                                              plib -Dccflags=-DAPPLLIB_EXP="/usr/local/lib/perl5/5.8.8/BSDPAN" -Doptimize=-O2 -fno-strict-al                                                                                                                                              iasing -pipe  -Ud_dosuid -Ui_gdbm -Dusethreads=n -Dusemymalloc=y -Duse64bitint'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef use5005threads=undef useithreads=undef usemultiplicity=undef
    useperlio=define d_sfio=undef uselargefiles=define usesocks=undef
    use64bitint=define use64bitall=undef uselongdouble=undef
    usemymalloc=y, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-DAPPLLIB_EXP="/usr/local/lib/perl5/5.8.8/BSDPAN" -DHAS_FPSETMASK -DHAS                                                                                                                                              _FLOATINGPOINT_H -fno-strict-aliasing -pipe -Wdeclaration-after-statement',
    optimize='-O2 -fno-strict-aliasing -pipe ',
    cppflags='-DAPPLLIB_EXP="/usr/local/lib/perl5/5.8.8/BSDPAN" -DHAS_FPSETMASK -DHAS_FLOATING                                                                                                                                              POINT_H -fno-strict-aliasing -pipe -Wdeclaration-after-statement'
    ccversion='', gccversion='3.4.4 [FreeBSD] 20050518', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    ivtype='long long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -Wl,-E'
    libpth=/usr/lib
    libs=-lm -lcrypt -lutil
    perllibs=-lm -lcrypt -lutil
    libc=, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='  -Wl,-R/usr/local/lib/perl5/5.8                                                                                                                                              .8/mach/CORE'
    cccdlflags='-DPIC -fPIC', lddlflags='-shared '


Characteristics of this binary (from libperl):
  Compile-time options: MYMALLOC PERL_MALLOC_WRAP USE_64_BIT_INT
                        USE_LARGE_FILES USE_PERLIO
  Locally applied patches:
        defined-or
  Built under freebsd
  Compiled at Nov  8 2007 08:36:03