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$writer->endTag("manager xmlns:xsi=http://www.w3.org/2001/XMLSchema-in
+stance");

$writer->startTag("manager xmlns:xsi=http://www.w3.org/2001/XMLSchema-
+instance");
.....
...
....
$writer->endTag("manager");
$writer->end;

NAMESPACES

...
      $writer->startTag([$rdfns, "Description"]);

    The first member of the array is a namespace URI, and the second p
+art is the local part of a qualified name. The module will automatica
+lly generate appropriate namespace declarations and will replace the 
+URI part with a prefix.

my $rdfns = "http://www.w3.org/1999/02/22-rdf-syntax-ns#";
my $writer = new XML::Writer(OUTPUT => \$output, NAMESPACES => 1);
...
print $output;