Here is a simple way to do it using XML::Twig:

use strict;
use warnings;
use XML::Twig;

my $xmlStr = <<XML;
<Invoice>

<InvoiceHeader>
<InvoiceType></InvoiceType>
<InvoiceStatus></InvoiceStatus>
<InvoiceNumber></InvoiceNumber>
<InvoiceDate></InvoiceDate>
<Supplier>
<Name> </Name>
<OrgNumber></OrgNumber>
<VatId></VatId>
</Supplier>
</InvoiceHeader>

<InvoiceDetails>

<BaseItemDetails>
<Description>desc1</Description>
<PerQuantity></PerQuantity>
<QuantityInvoiced></QuantityInvoiced>
<UnitOfMeasure></UnitOfMeasure>
<LineItemGrossAmount></LineItemGrossAmount>
<LineItemAmount></LineItemAmount>
<SuppliersProductId></SuppliersProductId>
<UnitPrice></UnitPrice>
<StartDate></StartDate>
<EndDate></EndDate>
</BaseItemDetails>

<BaseItemDetails>
<Description>desc2</Description>
<PerQuantity></PerQuantity>
<QuantityInvoiced></QuantityInvoiced>
<UnitOfMeasure></UnitOfMeasure>
<LineItemGrossAmount></LineItemGrossAmount>
<LineItemAmount></LineItemAmount>
<SuppliersProductId></SuppliersProductId>
<UnitPrice></UnitPrice>
<StartDate></StartDate>
<EndDate></EndDate>
</BaseItemDetails>

</InvoiceDetails>


</Invoice>
XML

my $twig= XML::Twig->new(
    twig_handlers => {
        'BaseItemDetails/Description' => 
            sub { print $_->text(), "\n" }
    }
);
$twig->parse($xmlStr);

__END__
desc1
desc2
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