You are trying to alter an attribute:

#!/usr/bin/perl
use strict;
use warnings;
use feature 'say';
use Mojo::DOM;

my $xml = '<header>
    <idset id="100">                            
        <a>item_a</a>
        <b>item_b</b>
    </idset>
</header>';

my $dom = Mojo::DOM->new->xml(1)->parse( $xml );
# say current attribute value
say 'Current value: ' . $dom->at('idset:first-of-type')->attr('id');
# change it to something
$dom->at('idset:first-of-type')->attr(id => 'derp');
# dump the whole DOM
say $dom->content;
[download]

Output:

Current value: 100
<header>
    <idset id="derp">                            
        <a>item_a</a>
        <b>item_b</b>
    </idset>
</header>
[download]