As it seems that mirod still did not have time to update those pages and to make the comparison more complete ... here's the solution using XML::Rules:

#!/usr/bin/perl -w
use strict;

use XML::Rules;

use FindBin qw($Bin);
use lib $Bin;
use wtr2_base;

my $DEBUG=0;

init_db();

my $parser = XML::Rules->new(
    start_rules => [
        Finvoice => sub {
            $_[1]->{errors} = []; # initialization
            reset_default_row_id();
            return 1; # necessary so that the tag is actually processe
+d!
        },
        'PaymentTermsDetails,VatSpecificationDetails,EpiDetails,Seller
+AccountDetails,BuyerPostalAddressDetails,SellerInformationDetails' =>
+ 'skip',
            # I don't care about those at all
    ],
    rules => [
        _default => 'content',
            # unless I say otherwise I'm only interested in the conten
+t

        'PaymentStatusDetails,BuyerCommunicationDetails,SellerPostalAd
+dressDetails,SellerCommunicationDetails' => 'pass no content',
            # I want to dissolve those into their parent tag. I'm not 
+interested in the whitespace around the child nodes

        'DeliveredQuantity,OrderedQuantity' => 'as is',
            # I need both the content and the attributes of those two

        BuyerPartyDetails => sub {
            delete $_[1]->{_content};
            check_buyer(  $_[1]->{BuyerPartyIdentifier}, $_[1]->{Buyer
+OrganisationName}, $_[3]->[0]{errors} );
            return $_[0] => $_[1];
        },
        OrderIdentifier => sub {
            check_po( $_[1]->{_content}, $_[3]->[0]{errors});
            return OrderIdentifier => $_[1]->{_content};
        },
        InvoiceRow => sub {
            delete $_[1]->{_content};
            $_[1]->{RowIdentifier} = default_row_id() unless $_[1]->{R
+owIdentifier};
            print "checking row $_[1]->{RowIdentifier}\n" if $DEBUG;
            check_qtty( $_[1]->{RowIdentifier},
                $_[1]->{DeliveredQuantity}->{_content},
                $_[1]->{DeliveredQuantity}->{QuantityUnitCode},
                $_[1]->{OrderedQuantity}->{_content},
                $_[1]->{OrderedQuantity}->{QuantityUnitCode},
                $_[3]->[0]{errors}
            );
            return '@invoicerow' => {
                row_id        => $_[1]->{RowIdentifier},
                sku           => $_[1]->{ArticleIdentifier},
                name          => $_[1]->{ArticleName},
                qty           => $_[1]->{DeliveredQuantity}->{_content
+},
                qty_unit      => $_[1]->{DeliveredQuantity}->{Quantity
+UnitCode},
                unit_price    => $_[1]->{UnitPriceAmount},
                amount_no_tax => $_[1]->{RowVatExcludedAmount},
                tax           => $_[1]->{RowVatAmount},
                amount        => $_[1]->{RowAmount},
            }
        },
        InvoiceDetails => sub {
            return invoice => {
                number         => $_[1]->{InvoiceNumber},
                date           => $_[1]->{InvoiceDate},
                po             => $_[1]->{OrderIdentifier},
                amount_no_tax  => $_[1]->{InvoiceTotalVatExcludedAmoun
+t},
                tax            => $_[1]->{InvoiceTotalVatAmount},
                amount         => $_[1]->{InvoiceTotalVatIncludedAmoun
+t},
            },
        },
        SellerPartyDetails => sub {
            return seller => {
                identifier      => $_[1]->{SellerPartyIdentifier},
                name            => $_[1]->{SellerOrganisationName},
                tax_code        => $_[1]->{SellerOrganisationTaxCode},
            }, address => {
                street         => $_[1]->{SellerStreetName},
                town           => $_[1]->{SellerTownName},
                zip            => $_[1]->{SellerPostCodeIdentifier},
                country_code   => $_[1]->{CountryCode},
                po_box         => $_[1]->{SellerPostOfficeBoxIdentifie
+r},
            }
        },

        Finvoice => sub {
            delete $_[1]->{_content};
            $_[1]->{invoice}{payment_status} = delete $_[1]->{PaymentS
+tatusCode};
            return
                invoice => $_[1]->{invoice},
                seller => $_[1]->{seller},
                address => $_[1]->{address},
                invoicerow => $_[1]->{invoicerow},
                errors => $_[1]->{errors},
                contact => {
                    name           => $_[1]->{SellerContactPersonName}
+,
                    phone          => $_[1]->{SellerPhoneNumberIdentif
+ier},
                    email          => $_[1]->{SellerEmailaddressIdenti
+fier},
                };
        },
    ],
);

my $filter = XML::Rules->new(
    style => 'filter',
    rules => [
        _default => 'raw',
        Finvoice => sub {
            push @{$_[1]->{_content}}, "  ", [errors => {error => $_[4
+]->{parameters}}], "\n";
            return ($_[0] => $_[1]);
        }
    ]
);

my @files= @ARGV || (<$dir{invoices}/*.xml>);

foreach my $file (@files) {
    print "Processing $file\n" if $DEBUG;
    my $data = $parser->parsefile($file);

    if (@{$data->{errors}}) {
        my $errors = $data->{errors};
        print "ERROR in $file\n  ", join( "\n  ", @$errors), "\n";
        my $rejected_file= rejected( $file);
        print "adding errors in $rejected_file\n" if( $DEBUG);
        open my $OUT, '>', $rejected_file or die "Can't open '$rejecte
+d_file' for writing: $^E\n";
        $filter->filterfile( $file, $OUT, $errors);
        close $OUT;

    } else {
        print "storing invoice $data->{invoice}->{number}\n";
        store_all( $data);
    }
}
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