$parameter_hash->{$this}{$value}{sort_by} = \&custom_sort;
... later, in some other subroutine ...
my $sort_routine = $parameter_hash->{$this}{$value}{sort_by} || #other standard sorts here;
foreach (sort $sort_routine keys %{ $parameter_hash->{$this}{$value}{value_list} }) {
  #do stuff
}