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$parameter_hash->{$this}{$value}{sort_by} = \&custom_sort;
... later, in some other subroutine ...
...
foreach (sort $sort_routine keys %{ $parameter_hash->{$this}{$value}{v
+alue_list} }) {
  #do stuff
}

foreach (sort $sort_routine->(\%href) keys %{ $somehashref }) {
  #do stff
}

sort &{ $sort_routine }(\%href);
sort &{ $sort_routine(\%href) };