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pdl> p $x
[
...
 [0 1 2]
 [6 7 8]
]

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

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

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

my %lookup = (
  0 => "0 1 2",
...

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