strange behaviour with 'use open IN => ":byte"'

If I run this script:

#!/usr/bin/perl
use open IN  => ":byte";
use constant A => 1;

on a MacBook Pro (Intel)

With perl version 5.8.6 (the pre-installed)

I get this error:

Can't locate constant.pm in @[EMAIL PROTECTED]
 (@[EMAIL PROTECTED]
 contains: /sw/lib/perl5
/sw/lib/perl5/darwin /System/Library/Perl/5.8.6/darwin-thread-multi-2level
/System/Library/Perl/5.8.6 /Library/Perl/5.8.6/darwin-thread-multi-2level
/Library/Perl/5.8.6 /Library/Perl
/Network/Library/Perl/5.8.6/darwin-thread-multi-2level
/Network/Library/Perl/5.8.6 /Network/Library/Perl
/System/Library/Perl/Extras/5.8.6/darwin-thread-multi-2level
/System/Library/Perl/Extras/5.8.6 /Library/Perl/5.8.1 .) at ./tst.pl line
3.

if I change the line to:
use open IN  => ":encoding(UTF16-BE)";

I get this error:
Unrecognized character \xE0 at /System/Library/Perl/5.8.6/constant.pm line
1.

In both cases:
If I swap the lines, everything runs smoothly.

Anyone seen this behaviour?
What to do about it?

Thanks
Bernhard

Summary of my perl5 (revision 5 version 8 subversion 6) configuration:
  Platform:
    osname=darwin, osvers=8.0, archname=darwin-thread-multi-2level
    uname='darwin b19.apple.com 8.0 darwin kernel version 8.3.0: mon oct 3
20:04:04 pdt 2005; root:xnu-792.6.22.obj~2release_ppc power macintosh
powerpc '
    config_args='-ds -e -Dprefix=/usr -Dccflags=-g  -pipe 
-Dldflags=-Dman3ext=3pm -Duseithreads -Duseshrplib'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=define use5005threads=undef useithreads=define
usemultiplicity=define
    useperlio=define d_sfio=undef uselargefiles=define usesocks=undef
    use64bitint=undef use64bitall=undef uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-g -pipe -fno-common -DPERL_DARWIN -no-cpp-precomp
-fno-strict-aliasing -I/usr/local/include',
    optimize='-O3',
    cppflags='-no-cpp-precomp -g -pipe -fno-common -DPERL_DARWIN
-no-cpp-precomp -fno-strict-aliasing -I/usr/local/include'
    ccversion='', gccversion='4.0.1 (Apple Computer, Inc. build 5363)',
gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t',
lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='env MACOSX_DEPLOYMENT_TARGET=10.3 cc', ldflags ='-L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib
    libs=-ldbm -ldl -lm -lc
    perllibs=-ldl -lm -lc
    libc=/usr/lib/libc.dylib, so=dylib, useshrplib=true,
libperl=libperl.dylib
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=bundle, d_dlsymun=undef, ccdlflags=' '
    cccdlflags=' ', lddlflags='-bundle -undefined dynamic_lookup
-L/usr/local/lib'


Characteristics of this binary (from libperl):
  Compile-time options: MULTIPLICITY USE_ITHREADS USE_LARGE_FILES
PERL_IMPLICIT_CONTEXT
  Locally applied patches:
        23953 - fix for File::Path::rmtree CAN-2004-0452 security issue
        33990 - fix for setuid perl security issues
        SPRINTF0 - fixes for sprintf formatting issues - CVE-2005-3962
  Built under darwin
  Compiled at Nov  1 2006 17:07:31
  %ENV:
    PERL5LIB="/sw/lib/perl5:/sw/lib/perl5/darwin"
  @[EMAIL PROTECTED]
    /sw/lib/perl5
    /sw/lib/perl5/darwin
    /System/Library/Perl/5.8.6/darwin-thread-multi-2level
    /System/Library/Perl/5.8.6
    /Library/Perl/5.8.6/darwin-thread-multi-2level
    /Library/Perl/5.8.6
    /Library/Perl
    /Network/Library/Perl/5.8.6/darwin-thread-multi-2level
    /Network/Library/Perl/5.8.6
    /Network/Library/Perl
    /System/Library/Perl/Extras/5.8.6/darwin-thread-multi-2level
    /System/Library/Perl/Extras/5.8.6
    /Library/Perl/5.8.1
    .