A complete working sample using Data::Dumper where an object having a DateTime reference can be printed in short and long versions depending on $Data::Dumper::Freezer:

# Foo.pm
package Foo;
use strict;
use warnings;
use DateTime;

sub new {
    my $class = shift;
    my $self = { _timevalue => DateTime->now, };
    bless( $self, $class );
    return $self;
}

# Modify _timevalue and return blessed reference
sub _dumper_hook {
    $_[0] = bless {
        %{ $_[0] },
        _timevalue => $_[0]->{_timevalue}->ymd . q{ }
          . $_[0]->{_timevalue}->hms,
      },
      ref( $_[0] );
}

1;
__END__
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Test:

# Foo.t:

use strict;
use warnings;
use Data::Dumper;
use Test::More;

plan tests => 2;

use Foo;
my $f = Foo::->new;
{
    local $Data::Dumper::Freezer = '_dumper_hook';
    like(
        Dumper($f),
        qr/\d{4}-\d{2}-\d{2}\s\d{2}:\d{2}:\d{2}/,
        q{prints short version}
    );
}

unlike(
    Dumper($f),
    qr/\d{4}-\d{2}-\d{2}\s\d{2}:\d{2}:\d{2}/,
    q{prints long version}
);

__END__
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Run:

$ perl Foo.t
1..2
ok 1 - prints short version
ok 2 - prints long version
$
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