I will try later to build perl & DateTime outside of Ports, but does not look like that would help any given the evidence so far.

It's not exactly the same as what you are doing, but I've just built this -DDEBUGGING perl-5.34.0 on my FreeBSD-12.0 system:

$ perl -V
Summary of my perl5 (revision 5 version 34 subversion 0) configuration
+:

  Platform:
    osname=freebsd
    osvers=12.0-release
    archname=amd64-freebsd
    uname='freebsd sisbsd.home 12.0-release freebsd 12.0-release r3416
+66 generic amd64 '
    config_args='-des -Duse64bitall -Dprefix=/home/sisyphus/perl-5.34.
+0-d-DEBUG -DDEBUGGING'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=undef
    usemultiplicity=undef
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
  Compiler:
    cc='cc'
    ccflags ='-DHAS_FPSETMASK -DHAS_FLOATINGPOINT_H -DDEBUGGING -fno-s
+trict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D
+_FORTIFY_SOURCE=2'
    optimize='-O2 -g'
    cppflags='-DHAS_FPSETMASK -DHAS_FLOATINGPOINT_H -DDEBUGGING -fno-s
+trict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion=''
    gccversion='4.2.1 Compatible FreeBSD Clang 6.0.1 (tags/RELEASE_601
+/final 335540)'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='cc'
    ldflags ='-Wl,-E  -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/lib /usr/local/lib /usr/lib/clang/6.0.1/lib
    libs=-lpthread -lgdbm -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -ldl -lm -lcrypt -lutil -lc
    libc=
    so=so
    useshrplib=false
    libperl=libperl.a
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags=' '
    cccdlflags='-DPIC -fPIC'
    lddlflags='-shared  -L/usr/local/lib -fstack-protector-strong'


Characteristics of this binary (from libperl):
  Compile-time options:
    DEBUGGING
    HAS_TIMES
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    USE_64_BIT_ALL
    USE_64_BIT_INT
    USE_LARGE_FILES
    USE_LOCALE
    USE_LOCALE_COLLATE
    USE_LOCALE_CTYPE
    USE_LOCALE_NUMERIC
    USE_LOCALE_TIME
    USE_PERLIO
    USE_PERL_ATOF
  Built under freebsd
  Compiled at Jul 26 2021 10:50:16
  @INC:
    /home/sisyphus/perl-5.34.0-d-DEBUG/lib/site_perl/5.34.0/amd64-free
+bsd
    /home/sisyphus/perl-5.34.0-d-DEBUG/lib/site_perl/5.34.0
    /home/sisyphus/perl-5.34.0-d-DEBUG/lib/5.34.0/amd64-freebsd
    /home/sisyphus/perl-5.34.0-d-DEBUG/lib/5.34.0
[download]

The Configure args that I specified (config_args) are as shown in that output. Having tested and installed that perl, I prepended its location to $PATH and ran cpan -i DateTime
That ran successfully, and I can find no problem in running DateTime-1.54:

$ perl -MDateTime -wle 'print $DateTime::VERSION;'
1.54
[download]

Cheers,
Rob