If it's pathalogical, what would be nonpathological?

$param->{'key'}          # { key => $key, value => $value }
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is slightly clearer than

$param->[0]              # [ $key, $value ]
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and much more clearer than

(keys(%$param))[0]       # { $key => $value }
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Update: I missed this:

The non-pathological representation would have: $hash{$key}=[$value1, $value2, $value3,...]

But it's not true. That doesn't maintain order, so the question remains. What would be nonpathological?

"That doesn't maintain order"

The array has the order.

Really? Please recreate the original from

{
   a=>[1,3,5],
   b=>[2,4,6],
}
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If you answered the following, you would be wrong.

a=>1
a=>3
a=>5
b=>2
b=>4
b=>6
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As far as you know, the original order was

b=>2
b=>4
a=>1
b=>6
a=>3
a=>5
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'C':
#define X_coord  0
#define Y_coord  1
#define MAX_ROWS 3

/* graph is a 2-d array of ints
   each row has an X and a Y coordinate
   Normally graph would be built dynamically as an
   array of pointer to array. The same [x][y]
   syntax can be used like in a predefined array.  

   The same [x][y] syntax works in Perl's array of
   references to arrays also. Although in most cases,
   I would consider this a "not good thing". 
*/   
 
for (int row =0; row <MAX_ROWS; row++)
{
   int this_pointX = graph[row][X_coord];
   int this_pointY = graph[row][Y_coord];
   ....
)   
##########
Perl: using Array of Array, about the same as 'C'
      this UGLY...

my $X_coord = 0;
my $Y_coord = 1;
foreach my $i (0..@graph-1) # forget 0..$#graph
                            # needless use of special variable 
{
   my $this_pointX = $graph[$i][$X_coord];
   my $this_pointY = $graph[$i][$Y_coord];
}
##########
Perl: Better than above using Array of Array:
foreach my $row (@graph)
{
   my ($this_pointX, $this_pointY) = @$row;
   ....
}
##########
Perl:
using Array of Hash:
foreach my $point (@graph) # graph is Array of Hash
                           # meaning an array of pointers
                           # to hash tables
{
   my $this_pointX = $point->{'X_coord'};
   my $this_pointY = $point->{'Y_coord'};
   ....
}
# there can be big advantages to avoiding this
# position dependent [$index] stuff as well as the
# need to pre-declare those indicies, The Perl AoH
# works very well to represent some types of C 2-D arrays.
# It is very easy to imagine how an implementer would
# go in that direction.
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I'm wasn't able to locate in that example any occurrences of the keys 'key' and 'value'.

I thought it was pretty clear: "In the below, imagine "key" being X and "value" being Y coordinates". I'm sorry that you didn't understand that. I was showing how this technique and data structure relates to other problems.