The first time you assign a value to a new hash, it gets allocated 8 slots. When the number of slots used reaches 6 (which may be more than 6 keys if you get hash collisions), it expands to 16 slots. And it doubles again when 3/4s of the slots (12/16) are used. And so on.

You can see this if you run the following code:

perl -wE"my %h;$h{$_}=$_, say $_, ':',scalar %h for 1 .. 1025" | more
1:1/8
2:2/8
3:3/8
4:3/8  ## collision
5:4/8
6:5/8
7:6/8
8:7/16
9:8/16
10:9/16
11:10/16 
12:10/16 ## collision
13:10/16 ## collision
14:11/16
15:11/16 ## collision
16:14/32
17:14/32 ## collision
...
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