Somewhat belated, but I had fun exploring different methods. Whether this is better in any way I doubt, but somebody might find it interesting.

#! perl -slw
use strict;
use Data::Dumper;

sub unwindDependancies{
    my ($hashref) = @_;
    my %copy = %{$hashref};
    my @keys = keys %$hashref;
    my @deps;

    for (@keys)  {
        next unless exists $copy{$_};
        push @deps, [ $_ ];
        while( exists $hashref->{$_}) {
            unshift( @{ $deps[-1] }, $_ = $hashref->{$_} );
            @{$deps[-1]} > @keys and die('Circular reference found inv
+olving:', "@{$deps[-1]}");
            delete $copy{$_};
        }
    }

    my %h;
    for (@deps) {
        my $r = \%h;
        $r = exists $r->{$_} ? $r->{$_} : ($r->{$_} = {}) for @$_;
    }
    #print Dumper \%h;

    my (@order, @stack);
    my $r=\%h;
    my $t=0;
    {
        while(my ($key, $val) = each %$r) {
            push @order, $key;
            push(@stack, $r), $r = $val, next if keys %$val;
        }
        $r = pop(@stack), redo if @stack;
    }
#    print "@order";
    return @order;
}

my %test = ( qw[A B  B C  D E  E F  F G  G C  J H  H F  I H  K H  N M 
+ O P  P Q  Q N  S U  T V]);
print join' ', unwindDependancies \%test;

my %replace = ( qw/
   COM  SEC
   MOB  HIS
   MOC  COM
   ICM1 INO
   ICM2 INO
   EU  HIS
   CY  GRE
   AE  MOB
   IN  ICC
   GR  GRE
   MH  MOC
   CO  MOC
   MO  HIS
/ );

print join' ', unwindDependancies \%replace;

__DATA__
C:\test>238721
V T C G F H I J K E D B A U S M N Q P O
ICC IN SEC COM MOC CO MH HIS EU MO MOB AE GRE CY GR INO ICM1 ICM2
[download]