package PpDumper;
# PpDumper is a subclass of Data::Dumper, which provides 
# a custom _dump() method
use Data::Dumper;
our @ISA='Data::Dumper';
sub _dump {
   my ($self, $val, $name) = @_;
   print "my _dump called\n";
   (ref($val) && $val->can('pp'))
   ? $val->pp($name)
   : $self->SUPER::_dump($val,$name);
}

sub PpDumper::Dumper {
  my ($class,@args) = @_;
  print "my own Dumper called\n";
  $class->SUPER::Dumper(@args);
}

package main;

....
my $h = { ... }
{
  local $Data::Dumper::Useperl=1;
  print("3:\n",PpDumper->Dumper($h),"\n");
}