I was anticipating a sub generator of some sort, where the hash and comparison function are passed in, and a new anonymous sub is returned. This sub could then be used in the code block position of the sort command.

Perhaps something like this, which steals some ideas from Choroba's post, though I've not tried it:

sub make_sort_sub {
    my $coderef = shift;
    my $hashref = shift;
    my $sort_sub = sub { something goes here }
    return $sort_sub;
}

my %hash = ( a => 5, b => 4, c => 3 );
my $keys_by_value = make_sort_sub(
    { $hashref->{$a} <=> $hashref->{$b} },
    \%hash);
my @keys_sorted_by_value = sort $keys_by_value %hash;
[download]

Then $keys_by_value can be reused elsewhere on the same hash. But can't be used on a different hash. :(

Hmmm, I'm thinking there's not a really elegant solution.