#!/usr/bin/perl
use warnings;
use strict;

use XML::LibXML;
use XML::LibXML::Reader;  
use Data::Dumper;

my $hash_ref;

my $reader = XML::LibXML::Reader->new( IO => *DATA );

while ( $reader->nextElement( 'Service' )) {
    my $number = $reader->getAttribute( 'Num');

    $reader->nextElement( 'Suppliermodules' );
    while ( $reader->nextElement( 'Hardwaremodule' )) {
        my $module = {};
        $module->{type} = $reader->getAttribute('Type');
        $module->{nr} = $reader->getAttribute('Nr');
        push @{$hash_ref->{$number}}, $module;
    }
}

print Dumper $hash_ref;
__DATA__
<?xml version="1.0" encoding="UTF-8"?>

<Servicelist xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="file:///files/service.xsd">

<Service Num="B7a" Name="temperature sensor">

<Des>It delivers actual temperature in the form ov Volts</Des>

<Customermodules>

   <Softwaremodule Service="ADC" Path="/main/ADCservice.xml"/>

</Customermodules>

<Suppliermodules>

   <Softwaremodule Service="input" Path="/main/inputservice.xml"/>
   <Softwaremodule Service="signal" Path="/main/signalservice.xml"/>
   <Hardwaremodule Type="engine"  Nr="18" Servicenum="1" Path="/main/engineservice.xml"/>
   <Hardwaremodule Type="motor"   Nr="7" Servicenum="1" Path="/main/motorservice.xml"/> 
   <Hardwaremodule Type="supply" Nr="1" Servicenum="1" Path="/main/supplyservice.xml"/>

</Suppliermodules>

</Service>
</Servicelist>