The wonderful thing about Perl is that we have the source code. So you can answer this question in the privacy of your own cubicle. First, let's find /dev/urandom:

$ grep /dev/urandom **/*.c
util.c:4550:#  define PERL_RANDOM_DEVICE "/dev/urandom"
[download]

OK, now we look in there, and we see that it just tries opening it if it can:

/* This test is an escape hatch, this symbol isn't set by Configure. *
+/
#ifndef PERL_NO_DEV_RANDOM
#ifndef PERL_RANDOM_DEVICE
   /* /dev/random isn't used by default because reads from it will blo
+ck
    * if there isn't enough entropy available.  You can compile with
    * PERL_RANDOM_DEVICE to it if you'd prefer Perl to block until the
+re
    * is enough real entropy to fill the seed. */
#  define PERL_RANDOM_DEVICE "/dev/urandom"
#endif
    fd = PerlLIO_open(PERL_RANDOM_DEVICE, 0);
    if (fd != -1) {
        if (PerlLIO_read(fd, (void*)&u, sizeof u) != sizeof u)
        u = 0;
    PerlLIO_close(fd);
    if (u)
        return u;
    }
#endif
[download]

So there's your answer. You can't know. It's not a configure constant. It just tries it, and if it fails, it doesn't use it.