Learning Perl would be a good book to start with. You may find that the monastery's Tutorials section is good for a quick start.

Here's a starting point for your XML manipulation using XML::TreeBuilder:

use strict;
use warnings;
use XML::TreeBuilder;

my $xml = <<XML;
<author-group>
<auth> </auth> <auth> </auth> <auth> </auth>
<aff> </aff> <aff> </aff>
<corres id="cor1"> </corres>
</author-group>
XML

my $root = XML::TreeBuilder->new ();

$root->parse ($xml);

for my $group ($root->look_down ('_tag', 'author-group')) {
    my @auths = $group->look_down ('_tag', 'auth');
    my $newRoot = HTML::Element->new ('xxx');
    
    $_->detach () for @auths;
    $newRoot->push_content (@auths);
    $group->unshift_content ($newRoot);
}

print $root->as_XML ();
[download]

Prints:

<author-group><xxx><auth> </auth><auth> </auth><auth> </auth></xxx>
  
<aff> </aff> <aff> </aff>
<corres id="cor1"> </corres>
</author-group>
[download]

Still pretty trivial eh.;)