UPDATE: I FOUND THE BUG!

I should not have called $class->SUPER::Dumper(@args);, but $class->SUPER::Dump(@args);, because Dumper() always blesses into a Data::Dumper object.

To my surprise, I found it more difficult to override _dump then expected. Maybe you can spot the error? I started like this:

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");
}
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When I run this, I see in the output of course my own Dumper called, but I don't see my _dump called being printed. This means that my custom _dump routine has not been called. This is strange: I verified that Data::Dumper::new (which is eventually called by Data::Dumper::Dumper) uses the two-argument form of bless, so my object should be of type PpDumper. Also, setting Useperl to 1 should ensure that no compiled components get in my way (I don't know whether this is necessary for overriding methods, so I put it there for the safe side).

Where is the bug?