The following should get you started. Note that there is a lack of error checking in the sample code!

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

my $content = do {local $/; <DATA>};
my $twig = XML::Twig->new(pretty_print => 'nice');

$twig->parse($content);

my @first = $twig->get_xpath('//one');
my @elts = $twig->get_xpath('//two');

push @elts, $twig->get_xpath('//three');

my $group = XML::Twig::Elt->new ('group');

$group->paste ('after', $first[0]);
$_->move ('last_child', $group) for @elts;

$content = $twig->sprint;
print $content;

__DATA__
<root>
<one>some text</one>
<two>some text</two>
<three>some text</three>
<four>more text</four>
</root>
[download]

Prints:

<root>
<one>some text</one>
<group>
<two>some text</two>
<three>some text</three>
</group>
<four>more text</four>
</root>
[download]