#!/usr/bin/perl
use strict;
use warnings;

my %hash = map {$_ => undef} 'a' .. 'z';
my ($at_once, $cnt, @bot_n, %known) = (3, 0, (), ());

while (1) {
    while (my ($key, $val) = each %hash) {
        next if defined $val;
        if (exists $known{$key}) {
            $hash{$key} = $known{$key};
            next;
        }
        my $inserted;
        for (0 .. $#bot_n) {
            if ($key lt $bot_n[$_]) {
                splice @bot_n, $_, 0, $key;
                $inserted = 1;
                last;
            }
        }
        push @bot_n, $key if ! $inserted;
        pop @bot_n if @bot_n > $at_once;
    }
    last if ! @bot_n;
    %known = ();
    for (@bot_n) {
        $known{$_} = ++$cnt;
    }
    @bot_n = ();
}
use Data::Dumper;
print Dumper(\%hash);