@ is for attributes. Remove the whole predicate, Twig doesn't understand it anyway. Just extract the uniqueName element, compare it to the wanted id, and change the element:

#!/usr/bin/perl
use warnings;
use strict;

use XML::Twig;

my $xmlFile = 'demo.xml';
my $t = XML::Twig->new( keep_atts_order => 1,
                        pretty_print    => 'indented_a',
                        twig_handlers   => {
                            'myconfig/myobj/myattributes/attribB' => \
+&getfoo,
                                           },
                      );
$t->parsefile($xmlFile);
$t->print;

sub getfoo {
    my ($t, $e) = @_;
    my $id = $e->parent->parent->first_child_text('uniqueName');
    if ('objB' eq $id) {
        $e->set_text(' NEW ');
    }
}
[download]

For comparison, the same task in XML::XSH2:

open demo.xml ;
set /myconfig/myobj[uniqueName='objB']/myattributes/attribB 'NEW' ;
save :b ;
[download]