my %hash = (f => 'd', c => 'a', b => 'e');
my @keys_ordered_by_value = sort { $hash{$a} cmp $hash{$b} } keys %hash;
#                                            ^^^
#                                            \|/
#  to sort numeric values use <=> here -------+
#  to sort in reverse order swap $a and $b:  { $hash{$a} cmp $hash{$b} }

foreach my $key (@keys_ordered_by_value) {
    print "key = $key, value = $hash{$key}\n";
}