If you want to print all the info from the XML, than a better approach is to use the 'simplify' method of XML::Twig (or XML::Simple) to create a Perl data structure.

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

my $twig = new XML::Twig();

my $config = $twig->parsefile('invoice2.xml')->simplify();

print Dumper( $config);
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With XML::XPath I see only a more tedious way (it's true that I don't have much experience with it.)

Also I changed the xml a little:

<Invoices>
  <Invoice>
    <InvoiceHeader>
      <InvoiceType>IT1</InvoiceType>
      <Supplier>
        <Name>Sup1</Name>
        <OrgNumber>Org1</OrgNumber>
      </Supplier>
    </InvoiceHeader>
    <InvoiceDetails>
      <BaseItemDetails>
        <Description>description11</Description>
        <PerQuantity></PerQuantity>
      </BaseItemDetails>
    <BaseItemDetails>
      <Description>description12</Description>
      <PerQuantity>101</PerQuantity>
    </BaseItemDetails>
    </InvoiceDetails>
  </Invoice>
  <Invoice>
    <InvoiceHeader>
      <InvoiceType>IT2</InvoiceType>
      <Supplier>
        <Name>Sup1</Name>
        <OrgNumber>Org2</OrgNumber>
      </Supplier>
    </InvoiceHeader>
    <InvoiceDetails>
      <BaseItemDetails>
        <Description>description21</Description>
        <PerQuantity></PerQuantity>
      </BaseItemDetails>
    <BaseItemDetails>
      <Description>description21</Description>
      <PerQuantity>101</PerQuantity>
    </BaseItemDetails>
    </InvoiceDetails>
  </Invoice>
</Invoices>
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use strict;
use warnings;
use XML::XPath;
use XML::XPath::XMLParser;

my $xp = XML::XPath->new(filename => 'invoice2.xml');

my $nodeset = $xp->find('//Invoices/Invoice');

foreach my $node ($nodeset->get_nodelist) {
    my $it  = $node->find('InvoiceHeader/InvoiceType')->string_value;
    print "IT: $it\n";
    my $spn = $node->find('Supplier/Name')->string_value;
    print " SPN: $it\n";
    my $des = $node->find('InvoiceDetails/BaseItemDetails/Description'
+)->string_value;
    print " DES: $des\n";
    # ...
    # You got the idea, yes?
}
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Good coding, Stefan

Update: 'TwigRoots' option is not needed in the XML::Twig example. You may want to use 'forcearray', 'keyattr' or other options to get a consistent data structure.

Update2: Removed 'TwigRoots' option from the code.