my @pdu_nums = 3 .. 8;
if ( $order ) {
    @pdu_nums = reverse @pdu_nums;
}

# You could check that @nodes >= @pdu_nums here, too.

foreach my $pdu_num ( @pdu_nums ) {
    my $node = shift @nodes;
    unless ( $node ) {
        die "A horrible death!\n";
    }
    $map{ $node} = join '', $pdu, '[', $pdu_num, '}';
}