I get the proper result:

/usr/bin/perl

use strict;
use warnings;

use XML::Twig;


my $twig = XML::Twig->parse( map_xmlns => { 'http://www.w3.org/2001/XM
+LSchema' => 'xsd', }, \*DATA);

printf "\$twig->root->gi: %s\n", $twig->root->gi;
printf "\$twig->root->namespace: %s\n", $twig->root->namespace;

__DATA__
<schema xmlns="http://www.w3.org/2001/XMLSchema"><element name="alert"
+/></schema>
[download]

gives me:

$twig->root->gi: xsd:schema
$twig->root->namespace: http://www.w3.org/2001/XMLSchema
[download]

Which version of XML::Twig are you using? I get the results above with XML::Twig 3.32, the latest CPAN release.