From @dcollinsn

Similar to RT #128639, this only appears on MinGW.org and not MinGW-w64.org compilers. This also requires that USE_64_BIT_INT be set (and possibly also long doubles).

PROG​:
use warnings 'uninitialized';

# aelem + const
use constant foo => \0;
$SIG{__WARN__} = sub {
print STDERR
  $_[0] =~ /\$a\[([^]]+)]/ && $1 == foo
  ? "ok\n"
  : ("$1 != ",0+foo,"\n")
};
() = "$a[foo]";
undef $SIG{__WARN__};

# Multideref
() = "$a[140688675223280]";
EXPECTED​:
ok
Use of uninitialized value $a[140688675223280] in string at - line 15.
GOT​:
ok
Use of uninitialized value $a[-1568491792] in string at - line 15.
# Failed test 363 - 64-bit array subscripts at lib\warnings\9uninit line 2163

Configuration information​:

W​:\buildbot\windows1\win32-mingw-ld64\build>perl -V
Summary of my perl5 (revision 5 version 25 subversion 3) configuration​:

  Platform​:
  osname=MSWin32
  osvers=10.0
  archname=MSWin32-x86-multi-thread-64int-ld
  uname=''
  config_args='undef'
  hint=recommended
  useposix=true
  d_sigaction=undef
  useithreads=define
  usemultiplicity=define
  use64bitint=define
  use64bitall=undef
  uselongdouble=define
  usemymalloc=n
  bincompat5005=undef
  Compiler​:
  cc='gcc'
  ccflags =' -s -O2 -DWIN32 -DPERL_TEXTMODE_SCRIPTS -DPERL_IMPLICIT_CONTEXT -DPERL_IMPLICIT_SYS -D__USE_MINGW_ANSI_STDIO -fwrapv -fno-strict-aliasing -mms-bitfields'
  optimize='-s -O2'
  cppflags='-DWIN32'
  ccversion=''
  gccversion='4.9.3'
  gccosandvers=''
  intsize=4
  longsize=4
  ptrsize=4
  doublesize=8
  byteorder=12345678
  doublekind=3
  d_longlong=define
  longlongsize=8
  d_longdbl=define
  longdblsize=12
  longdblkind=3
  ivtype='long long'
  ivsize=8
  nvtype='long double'
  nvsize=12
  Off_t='long long'
  lseeksize=8
  alignbytes=8
  prototype=define
  Linker and Libraries​:
  ld='g++'
  ldflags ='-s -L"c​:\perl\lib\CORE" -L"C​:\MinGW\lib"'
  libpth=C​:\MinGW\lib
  libs=-lmoldname -lkernel32 -luser32 -lgdi32 -lwinspool -lcomdlg32 -ladvapi32 -lshell32 -lole32 -loleaut32 -lnetapi32 -luuid -lws2_32 -lmpr -lwinmm -lversion -lodbc32 -lodbccp32 -lcomctl32
  perllibs=-lmoldname -lkernel32 -luser32 -lgdi32 -lwinspool -lcomdlg32 -ladvapi32 -lshell32 -lole32 -loleaut32 -lnetapi32 -luuid -lws2_32 -lmpr -lwinmm -lversion -lodbc32 -lodbccp32 -lcomctl32
  libc=
  so=dll
  useshrplib=true
  libperl=libperl525.a
  gnulibc_version=''
  Dynamic Linking​:
  dlsrc=dl_win32.xs
  dlext=dll
  d_dlsymun=undef
  ccdlflags=' '
  cccdlflags=' '
  lddlflags='-mdll -s -L"c​:\perl\lib\CORE" -L"C​:\MinGW\lib"'

Characteristics of this binary (from libperl)​:
  Compile-time options​:
  HAS_TIMES
  HAVE_INTERP_INTERN
  MULTIPLICITY
  PERLIO_LAYERS
  PERL_COPY_ON_WRITE
  PERL_DONT_CREATE_GVSV
  PERL_HASH_FUNC_ONE_AT_A_TIME_HARD
  PERL_IMPLICIT_CONTEXT
  PERL_IMPLICIT_SYS
  PERL_MALLOC_WRAP
  PERL_OP_PARENT
  PERL_PRESERVE_IVUV
  USE_64_BIT_INT
  USE_ITHREADS
  USE_LARGE_FILES
  USE_LOCALE
  USE_LOCALE_COLLATE
  USE_LOCALE_CTYPE
  USE_LOCALE_NUMERIC
  USE_LOCALE_TIME
  USE_LONG_DOUBLE
  USE_PERLIO
  USE_PERL_ATOF
  Built under MSWin32
  Compiled at Jul 16 2016 20​:39​:06
  @​INC​:
  W​:/buildbot/windows1/win32-mingw-ld64/build/lib
  .

W​:\buildbot\windows1\win32-mingw-ld64\build>gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=c​:/mingw/bin/../libexec/gcc/mingw32/4.9.3/lto-wrapper.exe
Target​: mingw32
Configured with​: ../src/gcc-4.9.3/configure --build=x86_64-pc-linux-gnu --host=mingw32 --prefix=/mingw --disable-win32-registry --target=mingw32 --with-arch=i586 --enable-languages=c,c++,objc,obj-c++,fortran,ada --enable-static --enable-shared --enable-threads --with-dwarf2 --disable-sjlj-exceptions --enable-version-specific-runtime-libs --enable-libstdcxx-debug --with-tune=generic --enable-nls
Thread model​: win32
gcc version 4.9.3 (GCC)

--
Respectfully,
Dan Collins