Thank you for your hint, I felt pretty safe that there is no need to check / debug that code for I just used the example code from the CPAN website:

use XML::BMEcat;
use File::Basename;

my $BMEcat = XML::BMEcat->new();
$BMEcat->setOutfile("catalog.xml");
my $GroupSystem = $BMEcat->creatGroupSystem();
my $CatalogGroup = $GroupSystem->creatCatalogGroup("04");
[download]

But using perl -d gave some more input:

GroupSystem::creatCatalogGroup('GroupSystem=ARRAY(0xb3bf70)', 04) called at ./bme_export.pl line 11

Still I am desperated and lost but something new to investigate, thanks again.