I recall that perls from 5.8.3 or so have code in place to watch out for this sort of degenerate case

I think it's the HV_MAX_LENGTH_BEFORE_SPLIT, currently set to 14.

/*    hv.c   */

#define HV_MAX_LENGTH_BEFORE_SPLIT 14

...

Perl_hv_common( ... )
{
            ...

            while ((counter = HeNEXT(counter)))
                n_links++;

            if (n_links > HV_MAX_LENGTH_BEFORE_SPLIT) {
                /* Use only the old HvKEYS(hv) > HvMAX(hv) condition t
+o limit
                   bucket splits on a rehashed hash, as we're not goin
+g to
                   split it again, and if someone is lucky (evil) enou
+gh to
                   get all the keys in one list they could exhaust our
+ memory
                   as we repeatedly double the number of buckets on ev
+ery
                   entry. Linear search feels a less worse thing to do
+.  */
                hsplit(hv);
            }

            ...
}
[download]

(the comment seems to be a left-over from an ealier implementation, though...)