I'm not an expert regarding PDL, but it's safe to say that re-arranging 2d data into 1d is a very cheap operation. There is no data movement involved - it's just the creation of a new index to the existing data.

UPDATE
Just to give an impression:

#!/usr/bin/perl

use strict;
use warnings;
use feature 'say';
use PDL;

my $p2d = sequence(3, 3);
say $p2d;
my $p1d = $p2d->clump(0, 1);
say $p1d;
$p1d->set(4, 42);
say $p2d;
__DATA__
[
 [0 1 2]
 [3 4 5]
 [6 7 8]
]

[0 1 2 3 4 5 6 7 8]

[
 [ 0  1  2]
 [ 3 42  5]
 [ 6  7  8]
]
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