How you ever got that code to even compile with this line in it I don't get (quotes are missing and the semicolon should be a comma)

                               *[@att='type'] => \&changeTagAttrOrderH
+andler;
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But anyway, your problem might be that this line

        $rootElt->set_atts( { $att1 => $att1Val, $att2 => $att2Val, $a
+tt3 => $att3Val});
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makes an ordinary hash (not an IxHash) and passes it to the set_atts method, so the order of the elements is lost.

As a simple standalone example, this code gives the attributes in a fixed order but it doesn't if you comment out the line with the arrow

use warnings; use strict;
use XML::Twig; use Tie::IxHash;
my $tw = XML::Twig->new("keep_atts_order", 1);
my %atts;
tie %atts, Tie::IxHash::; # <-----
%atts = ("year", 2010, "month", 3, "day", 5);
$tw->set_root(XML::Twig::Elt->new("date", \%atts));
$tw->flush; print "\n";
__END__
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Alternately use the set_att method like this:

use warnings; use strict;                                             
+                             
use XML::Twig;                                                        
+                      
my $tw = XML::Twig->new("keep_atts_order", 1);
$tw->set_root(my $de = XML::Twig::Elt->new("date"));
$de->set_att("year", 2010, "month", 3, "day", 5); 
$tw->flush; print "\n";
__END__
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(But note that set_att does not delete the existing attributes, so you may need to call del_atts first.)