See for example the replies to Understanding the Schwartzian transform.

Can you please assign / push $_ value in array ?

I think tybalt89 was just trying to tie  %hash1 into a solution in line with the structures given in your OP. I, myself, don't understand the relation of the  %hash1 and  %hash2 hashes: they seem essentially identical. See johngg's post for another approach to trying to tie these two hashes together meaningfully.

Separating out the elements a bit, see if this is clearer:

c:\@Work\Perl\monks>perl -wMstrict -le
"my %hash1 = map { $_ => 1 } 1 .. 5;
 ;;
 my %hash2;
 $hash2{'1'}{key} = 'a';
 $hash2{'2'}{key} = 'b';
 $hash2{'3'}{key} = 'a';
 $hash2{'4'}{key} = 'a';
 $hash2{'5'}{key} = 'b';
 ;;
 my @sorted_top_level_hash1_keys =
   map  $_->[0],
   sort { $a->[1] cmp $b->[1] or $a->[0] <=> $b->[0] }
   map  [ $_, $hash2{$_}{'key'} ],
   keys %hash1
   ;
 print qq{sorted top-level %hash1 keys: (@sorted_top_level_hash1_keys)
+};
 ;;
 for my $stlh1k (@sorted_top_level_hash1_keys) {
   print qq{key $stlh1k and $hash2{$stlh1k}{key}}
   }
"
sorted top-level %hash1 keys: (1 3 4 2 5)
key 1 and a
key 3 and a
key 4 and a
key 2 and b
key 5 and b
[download]