You could use a closure:

use Carp;

sub make_by_value_sorter {
    my ($hash, $op) = @_;

    return sub ($$) { $hash->{$_[0]} <=> $hash->{$_[1]} }
        if $op eq '<=>';
    return sub ($$) { $hash->{$_[0]} cmp $hash->{$_[1]} }
        if $op eq 'cmp';

    croak("Unknown sort op '$op'");
}

my $hash = {
    bar => 1,
    foo => 2,
    baz => 3,
    fah => 4,
};

my $by_value = make_by_value_sorter($hash, '<=>');

my @a = sort $by_value keys %$hash; # Simple case
my @b = sort {
    substr($a, 0, 1) cmp substr($b, 0, 1)
    or $by_value->($a, $b) # As part of another sort sub
} keys %$hash;
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Cheers,
-Anomo