While not the best solution, in the meantime perhaps you could wrap it in a function that returns the information you want

use strict;
use XML::Generator;

sub get_ns {
    my $in = shift;
    my $ns = substr($in, 1, index($in, ":"));
    $in =~ s/<$ns/</g;
    return $in;
}

my $gen = XML::Generator->new(
    conformance => 'strict',
    escape      => 'always',
    pretty      => 2,
    encoding    => 'UTF-8',
    version     => '1.0',
    namespace   => [itunes => "http://www.itunes.com/dtds/podcast-1.0.
+dtd"],
);

print $gen->xmldecl('standalone' , undef);
print get_ns($gen->rss([itunes => "http://www.itunes.com/dtds/podcast-
+1.0.dtd"], {version => '2.0'})) . "\n";
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