Each element in the array is a reference to an anon hash. Loop over these references, then deference to get the keys to put into sort. output of sort is sorted keys. see below.

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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