To make it change, force perl to make more buckets by adding more items to the hash.

>perl -le "for ('a'..'z') { $h{$_}=1; print scalar %h }
1/8
2/8
3/8
4/8
5/8
6/8
7/8
8/16
9/16
10/16
11/16
12/16
13/16
14/16
15/16
16/32
17/32
18/32
19/32
20/32
21/32
22/32
23/32
24/32
25/32
26/32
[download]

Note that sometimes, buckets will be reused:

>perl -le "for ('a'..'z') { $h{$_ x 2}=1; print scalar %h }
1/8
2/8
3/8
4/8
5/8
6/8
7/8
8/16
9/16
10/16
11/16
12/16
13/16
14/16
15/16  <- before adding 'pp'
15/16  <- after  adding 'pp'
15/16
15/16
15/16
15/16
21/32
21/32
21/32
21/32
21/32
21/32
[download]

Note the duplicate '15/16's. 'pp' was added to an existing bucket because 'pp' hashed to the same value as another key.