use strict;
use warnings;
use Data::Dumper;
use Tie::Hash::Sorted;

my %sequence = (
   1345 => 10,
   123  => 20,
   500  => 30,
);

print Dumper \%sequence;

# and here's the part that makes ears bleed ...
tie my %sorted_sequence, 'Tie::Hash::Sorted',
     Hash         => \%sequence,
     Sort_Routine => sub {[sort {$a <=> $b} keys %{$_[0]}]},
;

print Dumper \%sorted_sequence;

$sorted_sequence{901} = 40;
$sorted_sequence{201} = 50;

print Dumper \%sorted_sequence;