If you let Perl grow the hash, this super degenerate case will be detected and Perl will add bits to the hash key. The num keys start at 8, then 16,32,64,etc. The 9th entry to same hash value with buckets =8 would re-gen the entire hash. Now, I suppose that some case can be generated where at each bit addition, the same thing not only occurs, but becomes harder for earlier versions of Perl to detect!
I think my general advice about checking these parms: #buckets used, #total buckets and #total entries is a good one when dealing with very large or performance sensitive hashes.