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my @pdu_nums = 3 .. 8;
if ( $order ) {
...
    }
    $map{ $node} = join '', $pdu, '[', $pdu_num, '}';
}

use List::MoreUtils qw( pairwise );

...
@pdu_nums = reverse @pdu_nums if $order;

%map = { $b => "${pdu}[${a}]" } pairwise @pdu_nums, @nodes;