I'm glad I looked into this again. It appears to be a bug (well, it is at least an "interesting" algorithm). Here is my test code: 

#!/usr/bin/perl -w
use strict;
my $key='a';
my %check=($key=>1); # Used to find keys that hash similarly
my %fill= %check;    # Used to show hash isn't expanded
my %grow= %check;    # Used to show when hash does expand
$|++;
while( 1 ) {
    $grow{$a}= $check{++$a}= 1;
    if(  1 == substr(%check,0,1)  ) {
        $fill{$a}= 1;
        print %grow." ".%check." ".%fill." ".keys(%fill).$/;
    } else {
        delete $grow{$a};
    }
    delete $check{$a};
}

[download]
and the start of the output: 
1/8 1/8 1/8 2
1/8 1/8 1/8 3
1/8 1/8 1/8 4
1/8 1/8 1/8 5
1/8 1/8 1/8 6
1/8 1/8 1/8 7
2/16 1/8 1/8 8
2/16 1/8 1/8 9
2/16 1/8 1/8 10
2/16 1/8 1/8 11
2/16 1/8 1/8 12
2/16 1/8 1/8 13
2/16 1/8 1/8 14
2/16 1/8 1/8 15
4/32 1/8 1/8 16

[download]
which indicates that it is the number of keys (not the number of full buckets) that determines whether the hash should be extended to include twice as many buckets. But, this check is only done when a previously empty bucket gets an item. (Sorry, I'd normally explain this in more detail but I'm still a bit pressed for time.)

I'll have to find some time to look at the source code...

Update: I submitted a patch to fix this but it was mostly not understood and so I'm pretty sure it didn't get included. *sigh*

Update: Many months later I tried again and managed to get the patch accepted that time.

        - tye (but my friends call me "Tye")

In reply to (tye)Re2: print scalar %hash... erm by tye
in thread print scalar %hash... erm by pileswasp

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