#!/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};
}