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

my $fileName = 'Conf.xml'

my $twig = XML::Twig->new(
    twig_roots => { StartPoint => \&ReplaceAtriValue },
    twig_print_outside_roots => 1,
);

sub ReplaceAtriValue {
    my ($twig, $startPoint) = @_;
    if ( '09' eq $startPoint->att('Hours') )
    {
        $startPoint->set_att( Hours => '12');
    }
    $startPoint->print();
}
$twig->parsefile($fileName);