If you want to know what happens on those platforms, I'd look at Perl's source code rather than try to conduct a poll.
PROCSELFEXE_PATH is /proc/self/exe (Linux) or /proc/curproc/file (BSDish) 
#ifdef HAS_PROCSELFEXE
/* This is a function so that we don't hold on to MAXPATHLEN
   bytes of stack longer than necessary
 */
STATIC void
S_procself_val(pTHX_ SV *sv, const char *arg0)
{
    char buf[MAXPATHLEN];
    int len = readlink(PROCSELFEXE_PATH, buf, sizeof(buf) - 1);

    /* On Playstation2 Linux V1.0 (kernel 2.2.1) readlink(/proc/self/e
+xe)
       includes a spurious NUL which will cause $^X to fail in system
       or backticks (this will prevent extensions from being built and
       many tests from working). readlink is not meant to add a NUL.
       Normal readlink works fine.
     */
    if (len > 0 && buf[len-1] == '\0') {
      len--;
    }

    /* FreeBSD's implementation is acknowledged to be imperfect, somet
+imes
       returning the text "unknown" from the readlink rather than the 
+path
       to the executable (or returning an error from the readlink).  A
+ny valid
       path has a '/' in it somewhere, so use that to validate the res
+ult.
       See http://www.freebsd.org/cgi/query-pr.cgi?pr=35703
    */
    if (len > 0 && memchr(buf, '/', len)) {
        sv_setpvn(sv,buf,len);
    }
    else {
        sv_setpv(sv,arg0);
    }
}
#endif /* HAS_PROCSELFEXE */

STATIC void
S_set_caret_X(pTHX) {
    dVAR;
    GV* tmpgv = gv_fetchpvs("\030", GV_ADD|GV_NOTQUAL, SVt_PV); /* $^X
+ */
    if (tmpgv) {
#ifdef HAS_PROCSELFEXE
        S_procself_val(aTHX_ GvSV(tmpgv), PL_origargv[0]);
#else
#ifdef OS2
        sv_setpv(GvSVn(tmpgv), os2_execname(aTHX));
#else
        sv_setpv(GvSVn(tmpgv),PL_origargv[0]);
#endif
#endif
    }
}

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In reply to Re^2: Poll: Is your $^X an absolute path? (argv[0]) by ysth
in thread Poll: Is your $^X an absolute path? by xdg

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