If your data is indeed stored in a hash you could do something like:

#!/usr/bin/perl -w
use strict;

my (%sortme, $k);

%sortme = (
        1 => undef,
        2 => 4,
        3 => 5,
        4 => 3,
        5 => 1,
        6 => 3
);

foreach $k ( sort {
                return -1 if !defined $sortme{$a};
                return 1 if !defined $sortme{$b};
                $sortme{$a} <=> $sortme{$b};
            } keys %sortme ) {
        print "$k -> $sortme{$k}\n";
}
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substituding the print statement with whatever is needed for your larger application. Note that my example works if two keys have the same dependency. And you'll get a warning about uninitialzed value when it tries to print $sortme{1};