I tried the obvious of taking the example for "map_xmlns" and seeing if I could substitute an empty prefix. The result was that it leaves the prefix unchanged:

#!/usr/bin/perl
#very slightly modified example lifted from the docs

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



my $t= XML::Twig->new( map_xmlns => {'http://www.w3.org/2000/svg' => "
+"},
                         twig_handlers => 
                           { 'svg:circle' => sub { $_->set_att( r => 2
+0) } },
                         pretty_print => 'indented', 
                       )
                  ->parse( '<doc xmlns:gr="http://www.w3.org/2000/svg"
+>
                              <gr:circle cx="10" cy="90" r="10"/>
                           </doc>'
                         )
                  ->print;
[download]

The output is:

<doc xmlns:gr="http://www.w3.org/2000/svg">
  <gr:circle cx="10" cy="90" r="10"/>
</doc>
[download]

If I put the "svg"in the example back in the hash, it works as in the example. If I put a space, I get a prefix that's a space and a colon. XML::LibXML::Element will let you turn off the namespace prefix element by element, but I'm too lazy right now to work through that one. Try poking around through the various XML::LibXML children.