Thanks again.   I actually looked it up in the sources now (guess I should've done so earlier), and I presume it's this part in Perl_gv_check(), gv.c:1251:

        /* performance hack: if filename is absolute and it's a standa
+rd
         * module, don't bother warning */
#ifdef MACOS_TRADITIONAL
#   define LIB_COMPONENT ":lib:"
#else
#   define LIB_COMPONENT "/lib/"
#endif
        if (file
            && PERL_FILE_IS_ABSOLUTE(file)
            && (instr(file, LIB_COMPONENT) || instr(file, ".pm")))
        {
            continue;
        }
        CopLINE_set(PL_curcop, GvLINE(gv));
#ifdef USE_ITHREADS
        CopFILE(PL_curcop) = (char *)file;    /* set for warning */
#else
        CopFILEGV(PL_curcop) = gv_fetchfile(file);
#endif
        Perl_warner(aTHX_ packWARN(WARN_ONCE),
            "Name \"%s::%s\" used only once: possible typo",
            HvNAME_get(stash), GvNAME(gv));
...

with PERL_FILE_IS_ABSOLUTE being defined as (for my platform):

#define PERL_FILE_IS_ABSOLUTE(f)        (*(f) == '/')
[download]

So, I removed the .pm extension (the other two criteria, i.e. /lib/ and absolute path, don't apply in my case -- I even checked with strace(1) what it's really loading). (Update: actually, taking a closer look, I see it's PERL_FILE_IS_ABSOLUTE(file) && ..., so the other two shouldn't apply anyway...)

Interestingly, still none of the following do issue the warning:

require "WarnMe";

---

do "WarnMe";

---

my $code = join '', <DATA>;
eval $code;
__DATA__
my $val = $Some::Nonexistent::Pkg::variable;

---

eval 'my $val = $Some::Nonexistent::Pkg::variable';
[download]

Well, I guess some other hack is playing tricks on me...   Anyway. :)