perl -V

I found the answer to my problem, even if I don't really understand why.

I uninstalled the ActivePerl I was using, ActivePerl-5.24.2.2403-MSWin32-x64-403863.exe and installed the 32-bit version ActivePerl-5.24.2.2403-MSWin32-x86-64int-403863.exe.

My code now works...but things look funny. My PERLLIB still needs to reference the Perl64 (empty) directory, as does @INC, but the installer used the Perl directory. The PATH is set correctly to C:\Perl\site\bin;C:\Perl\bin

I'm including the output of perl -V below.

Summary of my perl5 (revision 5 version 24 subversion 2) configuration
+:

  Platform:
    osname=MSWin32, osvers=6.1, archname=MSWin32-x86-multi-thread-64in
+t
    uname=''
    config_args='undef'
    hint=recommended, useposix=true, d_sigaction=undef
    useithreads=define, usemultiplicity=define
    use64bitint=define, use64bitall=undef, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='undef', ccflags =' -s -O2 -DWIN32 -DPERL_TEXTMODE_SCRIPTS -DUS
+E_SITECUSTOMIZE -DPERL_IMPLICIT_CONTEXT -DPERL_IMPLICIT_SYS -fwrapv -
+fno-strict-aliasing -mms-bitfields',
    optimize='-s -O2',
    cppflags='-DWIN32'
    ccversion='', gccversion='4.6.3', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=12345678
+, doublekind=3
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=1
+2, longdblkind=3
    ivtype='long long', ivsize=8, nvtype='double', nvsize=8, Off_t='lo
+ng long', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='undef', ldflags ='-s -static-libgcc -static-libstdc++ -L"C:\Pe
+rl\lib\CORE" -L"C:\MinGW\i686-w64-mingw32\lib"'
    libpth=C:\MinGW\i686-w64-mingw32\lib
    libs=-lmoldname -lkernel32 -luser32 -lgdi32 -lwinspool -lcomdlg32 
+-ladvapi32 -lshell32 -lole32 -loleaut32 -lnetapi32 -luuid -lws2_32 -l
+mpr -lwinmm -lversion -lodbc32 -lodbccp32 -lcomctl32
    perllibs=-lmoldname -lkernel32 -luser32 -lgdi32 -lwinspool -lcomdl
+g32 -ladvapi32 -lshell32 -lole32 -loleaut32 -lnetapi32 -luuid -lws2_3
+2 -lmpr -lwinmm -lversion -lodbc32 -lodbccp32 -lcomctl32
    libc=, so=dll, useshrplib=true, libperl=libperl524.a
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_win32.xs, dlext=dll, d_dlsymun=undef, ccdlflags=' '
    cccdlflags=' ', lddlflags='-mdll -s -static-libgcc -static-libstdc
+++ -L"C:\Perl\lib\CORE" -L"C:\MinGW\i686-w64-mingw32\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_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_PERLIO
                        USE_PERL_ATOF USE_SITECUSTOMIZE
  Locally applied patches:
        ActivePerl Build 2403 [403863]
  Built under MSWin32
  Compiled at Aug 25 2017 05:28:36
  %ENV:
    PERLLIB="C:/Perl64/site/lib/Devel;C:/Perl64/Digest;C:/Perl64/site/
+lib/LWP/Protocol;C:/Perl64/site/lib/Crypt;C:/Perl64/site/lib/SSLeay;C
+:/Perl64/site/lib/ExtUtils;C:/Perl64/site/lib/LWP;C;/Perl64/lib/ExtUt
+ils;C:/Perl64/lib/Crypt;C:/Perl64/lib/Crypt/SSLeay;c:/Perl64/lib/HTTP
+/UserAgent/Cookies;c:/Perl64/lib/ExtUtils/WWW/Mechanize"
  @INC:
    C:/Perl64/site/lib/Devel
    C:/Perl64/Digest
    C:/Perl64/site/lib/LWP/Protocol
    C:/Perl64/site/lib/Crypt
    C:/Perl64/site/lib/SSLeay
    C:/Perl64/site/lib/ExtUtils
    C:/Perl64/site/lib/LWP
    C
    /Perl64/lib/ExtUtils
    C:/Perl64/lib/Crypt
    C:/Perl64/lib/Crypt/SSLeay
    c:/Perl64/lib/HTTP/UserAgent/Cookies
    c:/Perl64/lib/ExtUtils/WWW/Mechanize
    C:/Perl/site/lib
    C:/Perl/lib
    .
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Your perllib needs to be empty as youre using it wrong

I concur. use constant is erroneously loading C:/Perl64/site/lib/ExtUtils/Constant.pm because of the junk you've added to @INC. Unset env var PERLLIB.