Oops - try this

sub {
my $fatherid = shift;
my $db = $DB->table('Category');
my $sth = $db->select({FatherID=>$fatherid});
my $output;
while (my $cat = $sth->fetchrow_hashref){
my $url=$DB->table('Category')->as_url($cat->{Full_Name);
$output.=qq~<li><a href="$CFG->{build_root_url}/$url">$cat->{Name}</a>~;
}
return $output;
}