Sorry, not a XML::Twig solution:

use strict;
use warnings;
use XML::Rules;

my $parser = XML::Rules->new(
    rules => [
        _default => 'raw',
        item => 'as array',
        orderedlist => sub {
            delete $_[1]->{_content}; # remove the whitespace

            for (my $inst = 1; $inst <= @{$_[1]->{item}}; $inst++) {
                push @{$_[1]->{_content}}, "\n", [inst => "\t$inst.\t"
+], [item => $_[1]->{item}[$inst-1]];
            }
            push @{$_[1]->{_content}}, "\n";
            delete $_[1]->{item};
            return $_[0] => $_[1];
        }
    ],
    style => 'filter',
);

$parser->filter(\*DATA);


__DATA__
<chapter>
<chbody>
<exercises att="problems">
<title>Content</title>
<orderedlist numeration="number">
<item><para>Text1</para></item>
<item><para>Text2</para></item>
<item><para>Text3</para></item>
</orderedlist>
<orderedlist numeration="number">
<item><para>Text1</para></item>
<item><para>Text2</para></item>
<item><para>Text3</para></item>
<item><para>Text4</para></item>
</orderedlist>
</exercises>
</chbody>
</chapter>
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