pdl> p $x
[
 [0 1 2]
 [3 4 5]
 [6 7 8]
 [0 1 2]
 [0 1 2]
 [6 7 8]
]

##</code><code>##

pdl> p uniq $x
[0 1 2 3 4 5 6 7 8]

##</code><code>##

pdl> p scalar hist $x,0,256,1
[3 3 3 1 1 1 2 2 2 0 0 0 0 0 ....]

##</code><code>##

[0 1 2] 3
[3 4 5] 1
[6 7 8] 2

##</code><code>##

my %lookup = (
  0 => "0 1 2",
  1 => "3 4 5",
  2 => "6 7 8",
);

# Then $x reduces down to
$x = [ 0 1 2 0 0 2 ];