use strict;
use warnings;
use XML::Twig;
use Data::Dumper;

my $xfile = <<EOF;
<?xml version="1.0" encoding="utf-8"?>
<install type="module" version="1.0.0">
    <name>TEST</name>
    <notebook_tabs>
        <tab index="1">Create
            <button 
            internal_name="test1" 
            parent="-1" 
            text="button 1" 
            x_coordinate="5" 
            y_coordinate="5" 
            x_size="-1" 
            y_size="-1" 
            evt_1="EVT_BUTTON" 
            evt_1_function="testclick" />

            <button 
            internal_name="test2" 
            parent="-1" 
            text="button 2" 
            x_coordinate="50" 
            y_coordinate="50" 
            x_size="-1" 
            y_size="-1" 
            evt_1="EVT_BUTTON" 
            evt_1_function="testclick2" />

        </tab>
        <tab index="2">Assign</tab>
    </notebook_tabs>
</install>
EOF

my $t= new XML::Twig();
$t->parse($xfile);
my $install = $t->root();
my $note_tabs = $install->first_child('notebook_tabs');
my @tabs = $note_tabs->children('tab');
for my $tab (@tabs) {
    print "\nTab ", $tab->att('index'), "\n";
    my @buttons = $tab->children('button');
    print "number of buttons = ", scalar @buttons, "\n";
    for my $button (@buttons) {
        my @params = $button->atts();
        print Dumper(\@params);
    }
}
[download]

Prints:

Tab 1
number of buttons = 2
$VAR1 = [
          {
            'parent' => '-1',
            'y_coordinate' => '5',
            'x_coordinate' => '5',
            'evt_1_function' => 'testclick',
            'x_size' => '-1',
            'evt_1' => 'EVT_BUTTON',
            'internal_name' => 'test1',
            'text' => 'button 1',
            'y_size' => '-1'
          }
        ];
$VAR1 = [
          {
            'parent' => '-1',
            'y_coordinate' => '50',
            'x_coordinate' => '50',
            'evt_1_function' => 'testclick2',
            'x_size' => '-1',
            'evt_1' => 'EVT_BUTTON',
            'internal_name' => 'test2',
            'text' => 'button 2',
            'y_size' => '-1'
          }
        ];

Tab 2
number of buttons = 0
[download]