Use something like this to create $by_sort_method:

sub create_sorter
{
       my $contents = shift;

       return sub {
               # sort however you want here
               $contents{$a}{FOO} cmp $contents{$b}{FOO}
       };
}

my $by_sort_method = create_sorter \%contents;

foreach my $id (sort $by_sort_method keys %contents) {
       ...
}
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