You don't need to dereference the array, because the array elements to compare will be stored in $a and $b. All you need to do is to extract the fields of your array elements and compare them. See, again, the examples in perlfaq4, and also maybe References Quick Reference. Maybe you want to show the code you have written and explain where you actually have the problems instead of trying to make us write your code for you.

use strict;
use warnings;
my @aoh =(
 {
    3 => 15,
    4 => 8,
    5 => 9,
 },
 {
    3 => 11,
    4 => 25,
    5 => 6,
 },
{
    3 => 5,
    4 => 18,
    5 => 5,
},
{
    0 => 16,
    1 => 11,
    2 => 7,
},
{
    0 => 21,
    1 => 13,
    2 => 31,
 },
{
    0 => 11,
    1 => 14,
    2 => 31,
},
);

foreach my $href (@aoh)
{
   my @sorted_keys = sort {$href->{$a} <=> $href->{$b} } keys %$href;
   foreach my $key (@sorted_keys)
   {
       print "$key => $href->{$key}\n"
   }
   print "\n";
}
__END__
4 => 8
5 => 9
3 => 15

5 => 6
3 => 11
4 => 25

3 => 5
5 => 5
4 => 18

2 => 7
1 => 11
0 => 16

1 => 13
0 => 21
2 => 31

0 => 11
1 => 14
2 => 31
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