There is an example in the pod for Data::Rmap that shows how to use it's rmap_to() function to traverse a structure maintaining local state (the path through the hashes). It could be used as a starting point to create a custom iterator function.

# Traverse a tree using localize state
 $tree = [
     one =>
     two =>
     [   
         three_one =>
         three_two =>
         [   
             three_three_one =>
         ],
         three_four =>
     ],
     four =>
     [   
         [   
             five_one_one =>
         ],
     ],
 ];

 @path = ('q');
 rmap_to {
     if(ref $_) {
         local(@path) = (@path, 1); # ARRAY adds a new level to the pa
+th
         $_[0]->recurse(); # does stuff within local(@path)'s scope
     } else {
         print join('.', @path), " = $_ \n"; # show the scalar's path
     }
     $path[-1]++; # bump last element (even when it was an aref)
 } ARRAY|VALUE, $tree;

 # OUTPUT
 # q.1 = one 
 # q.2 = two 
 # q.3.1 = three_one 
 # q.3.2 = three_two 
 # q.3.3.1 = three_three_one 
 # q.3.4 = three_four 
 # q.4 = four 
 # q.5.1.1 = five_one_one
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This would involve writing your own function that wrapped code something similar to the above, but localise the keys on the fly. You would pass a callback (function or block) to the wrapper function, and it would call your code, with the appropriate variables ($env, $platform, $host, $target etc.) set and localised. You write this once the call with different callbacks each time you need to iterate the structure. If this idea interests you, but you need a bit more info on implementing it /msg me.

The other thought that crossed my mind was if you only ever access this structure through iteration, rather than individual direct accesses, then a HoH is probably the wrong structure. An AoH would be easier to use in that case. It might look something like this (pseudo-code):

my @servers = (
     {
          type      => production | development,
          env       => Windows | unix | Database,
          hostname  => the hostname
          targets   => [
                name1 => [ #total, #free ].
                name2 => [ #total, #free ],
                ...
          ],
     },
     {
          type      => production | development,
          env       => Windows | unix | Database,
          hostname  => the hostname
          targets   => [
                name1 => [ #total, #free ].
                name2 => [ #total, #free ],
                ...
          ],
     },
     ...
);
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And to iterate it:

use constant { TOTAL => 0, FREE => 1 };

for my @server ( @servers ) {
    printf "Server: %s type:%s Env: %s\n", 
            $server->{ hostname },
            $server->{ type },
            $server->{ env };
    for my $target ( @{ $server->{ targets } } ) {
        printf "\tTotal: %d  Free: %d\n", @{ $target }[ TOTAL, FREE ];
    }
}
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