use Data::Dumper;
use DateTime;
use Data::Visitor::Callback;

my $glob = { dt => DateTime->now(),
             h  => { a => 1, b => 2, dtx => DateTime->now() }, };
my $v = Data::Visitor::Callback->new(
    'DateTime' => sub { $_ = "$_" },
    'ignore_return_values' => 1,
    );
$v->visit( $glob );

print Dumper $glob;
__END__
$VAR1 = { 
          'h' => { 
                   'dtx' => '2008-12-02T17:56:41',
                   'a' => 1,
                   'b' => 2
                 },
          'dt' => '2008-12-02T17:56:41'
        };