I can confirm this for Perl 5.8.4, compiled from source for Debian Linux:

perl5.8.4 -we 'BEGIN{my %x=()}'
[download]

produces

Bizarre copy of HASH in leavesub at -e line 1.
BEGIN failed--compilation aborted at -e line 1.
[download]

It seems that it is necessary that %x is a lexical variable local to the BEGIN block. The following other variants produce no error:

perl5.8.4 -we '{my %x=()}'
perl5.8.4 -we 'my %x=()'
perl5.8.4 -we 'BEGIN{ %x=()}'
Name "main::x" used only once: possible typo at -e line 1.
perl5.8.4 -we 'my%x;BEGIN{ %x=()}'
perl5.8.4 -we 'my%x;BEGIN{ {%x=()}}'
[download]

This seems to indicate that a possible workaround would be to move the lexical declaration out of the BEGIN block, but into an inner block, so that it gets cleared after the program has started running:

perl5.8.4 -we '{my%x;BEGIN{ {%x=()}}}'
[download]

Deparse produces the non-initializing code for the oneliner, so I guess that davido is correct with his diagnosis that the optimization causes this error:

perl5.8.4 -w -MO=Deparse -e 'BEGIN{ my %x=()}'
Bizarre copy of HASH in leavesub at -e line 1.
BEGIN failed--compilation aborted at -e line 1.
BEGIN { $^W = 1; }
sub BEGIN {
    my %x;
}
[download]

My Perl has the following characteristics:

corion@aliens:~$ perl5.8.4 -V
Summary of my perl5 (revision 5 version 8 subversion 4) configuration:
  Platform:
    osname=linux, osvers=2.4.25, archname=i686-linux-thread-multi
    uname='linux aliens 2.4.25 #2 sun feb 22 12:12:48 cet 2004 i686 un
+known '
    config_args='-Dinc_version_list=none -Dprefix=/opt/perl'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=define use5005threads=undef useithreads=define usemulti
+plicity=define
    useperlio=define d_sfio=undef uselargefiles=define usesocks=undef
    use64bitint=undef use64bitall=undef uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS 
+-fno-strict-aliasing -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE
+_OFFSET_BITS=64',
    optimize='-O3',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS -fno-stri
+ct-aliasing -I/usr/local/include'
    ccversion='', gccversion='2.95.4 20011002 (Debian prerelease)', gc
+cosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=1
+2
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t',
+ lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
    perllibs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
    libc=/lib/libc-2.2.5.so, so=so, useshrplib=false, libperl=libperl.
+a
    gnulibc_version='2.2.5'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fpic', lddlflags='-shared -L/usr/local/lib'


Characteristics of this binary (from libperl):
  Compile-time options: MULTIPLICITY USE_ITHREADS USE_LARGE_FILES PERL
+_IMPLICIT_CONTEXT
  Built under linux
  Compiled at May  8 2004 09:58:09
  @INC:
    /opt/perl/lib/5.8.4/i686-linux-thread-multi
    /opt/perl/lib/5.8.4
    /opt/perl/lib/site_perl/5.8.4/i686-linux-thread-multi
    /opt/perl/lib/site_perl/5.8.4
    /opt/perl/lib/site_perl
    .
[download]