BrowserUk,
Perhaps the following minor modifications are more to your liking:

#!/usr/bin/perl
use strict;
use warnings;
use Time::HiRes qw/gettimeofday tv_interval/;

my $items = $ARGV[0] || 100;
my $str = 'a';
my %hash = map {$str++ => undef} 1 .. $items;
my $at_once = int($items * .10);

my ($cnt, @bot_n, %known) = (0, (), ());

my $beg = [gettimeofday];
while (1) {
    while (my ($key, $val) = each %hash) {
        next if defined $val;
        if (exists $known{$key}) {
            $hash{$key} = $known{$key};
            next;
        }
        my $inserted;
        my ($min, $max) = (0, $#bot_n);
        while (1) {
            my $mid = $min + (($max - $min) / 2);
            last if ! defined $bot_n[$mid];
            $key gt $bot_n[$mid] ? ($min = $mid + 1) : ($max = $mid - 
+1);
            last if $min > $max;
        }
        splice @bot_n, $min, 0, $key;
        pop @bot_n if @bot_n > $at_once;
    }
    last if ! @bot_n;
    %known = ();
    $known{$_} = ++$cnt for @bot_n;
    @bot_n = ();
}
my $elapsed = tv_interval($beg, [gettimeofday]);
my $per = sprintf("%.7f", $elapsed / $items);
print "Took $elapsed seconds for $items items ($per per item)\n";
__DATA__
C:\tmp>perl buk.pl 100
Took 0.003766 seconds for 100 items (0.0000377 per item)

C:\tmp>perl buk.pl 1000
Took 0.054244 seconds for 1000 items (0.0000542 per item)

C:\tmp>perl buk.pl 10000
Took 0.734375 seconds for 10000 items (0.0000734 per item)

C:\tmp>perl buk.pl 100000
Took 12.515625 seconds for 100000 items (0.0001252 per item)

C:\tmp>perl buk.pl 1000000
Took 394.375 seconds for 1000000 items (0.0003944 per item)
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