Intriguing indeed. Especially as there is such close correspondance betwen our standard Perl runtimes, and such disparity with the autoboxed version. I'm at a loss ot explain it. (Could you try running with the command line options show below?).

Perhaps a few others could run the code--with emphasis on AS & non-AS Perl's on Windows; 32-bit & 64-bit; threaded & non-threaded--though you seem to have ruled that out. Maybe they would shed some light.

This is a direct, unedited c&p from my console running first the standard Perl version on 4000, and then the autoboxed version on 400-4000 showing the exponential growth in runtime:

C:\test>826456 -N=4000
4000: Took 1.367 seconds

C:\test>826456-a -N=400
400: Took 0.025 seconds

C:\test>826456-a -N=800
800: Took 0.988 seconds

C:\test>826456-a -N=1600
1600: Took 13.999 seconds

C:\test>826456-a -N=3200
3200: Took 119.228 seconds

C:\test>826456-a -N=4000
4000: Took 234.037 seconds
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My system is

C:\test>perl -V
Summary of my perl5 (revision 5 version 10 subversion 1) configuration
+:

  Platform:
    osname=MSWin32, osvers=5.2, archname=MSWin32-x64-multi-thread
    uname=''
    config_args='undef'
    hint=recommended, useposix=true, d_sigaction=undef
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=und
+ef
    use64bitint=define, use64bitall=undef, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cl', ccflags ='-nologo -GF -W3 -MD -Zi -DNDEBUG -Ox -GL -Wp64 
+-fp:precise -DWIN32 -D_CON
    optimize='-MD -Zi -DNDEBUG -Ox -GL -Wp64 -fp:precise',
    cppflags='-DWIN32'
    ccversion='15.0.21022', gccversion='', gccosandvers=''
    intsize=4, longsize=4, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=undef, longlongsize=8, d_longdbl=define, longdblsize=8
    ivtype='__int64', ivsize=8, nvtype='double', nvsize=8, Off_t='__in
+t64', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='link', ldflags ='-nologo -nodefaultlib -debug -opt:ref,icf -lt
+cg  -libpath:"C:\Perl64\li
    libpth=\lib
    libs=  oldnames.lib kernel32.lib user32.lib gdi32.lib winspool.lib
+  comdlg32.lib advapi32.li
    perllibs=  oldnames.lib kernel32.lib user32.lib gdi32.lib winspool
+.lib  comdlg32.lib advapi3
    libc=msvcrt.lib, so=dll, useshrplib=true, libperl=perl510.lib
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_win32.xs, dlext=dll, d_dlsymun=undef, ccdlflags=' '
    cccdlflags=' ', lddlflags='-dll -nologo -nodefaultlib -debug -opt:
+ref,icf -ltcg  -libpath:"C


Characteristics of this binary (from libperl):
  Compile-time options: MULTIPLICITY PERL_DONT_CREATE_GVSV
                        PERL_IMPLICIT_CONTEXT PERL_IMPLICIT_SYS
                        PERL_MALLOC_WRAP PL_OP_SLAB_ALLOC USE_64_BIT_I
+NT
                        USE_ITHREADS USE_LARGE_FILES USE_PERLIO
                        USE_SITECUSTOMIZE
  Locally applied patches:
        ActivePerl Build 1006 [291086]
        32728 64-bit fix for Time::Local
  Built under MSWin32
  Compiled at Aug 24 2009 13:45:20
  @INC:
    C:/Perl64/site/lib
    C:/Perl64/lib
    .

C:\test>perl -Mautobox -E"say $autobox::VERSION"
2.55

C:\test>perl -Mautobox::Core -E"say $autobox::Core::VERSION"
0.6
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Full console log including dumping sources to the screen: