Remember that keys != buckets. Sometimes, and more frequently than you might imagine especially with small hashes, two or more keys will hash to the same bucket.

That said, from a cursory inspection, it seems to be when the number of buckets in use exceeds 2/3rds of the buckets available:

++$h{$_ } and print scalar keys %h, ' : ', scalar %h for 'aaaa' .. 'zz
+zz';;

1  :  1/8
2  :  2/8
3  :  2/8           ## hash colision
4  :  2/8           ## hash colision
5  :  3/8
6  :  4/8
7  :  5/8           ## 5/8ths
8  :  6/16    
9  :  6/16
10  :  7/16 
11  :  7/16        ## hash colision
12  :  8/16
13  :  9/16
14  :  9/16        ## hash colision
15  :  9/16        ## hash colision
16  :  10/16       ## 5/8ths
17  :  15/32
18  :  15/32       ## hash colision
19  :  16/32
20  :  16/32       ## hash colision
21  :  16/32       ## hash colision
22  :  17/32
23  :  17/32       ## hash colision
24  :  18/32
25  :  19/32
26  :  19/32       ## hash colision
27  :  20/32
28  :  20/32       ## hash colision
29  :  20/32       ## hash colision
30  :  20/32       ## hash colision ## 5/8ths
31  :  21/32
32  :  25/64
...
60  :  40/64   
61  :  40/64       ## hash colision
62  :  40/64       ## hash colision 5/8ths
63  :  41/64       ##  2/3rds
64  :  51/128
...
128  :  85/128     ## 2/3rds
129  :  106/256
130  :  107/256
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