But if I could skip this level of indirection I will be happy.

Well, you can hide it using AUTOLOAD

# untested

sub AUTOLOAD {
  if (my ($pkg, $name) = $AUTOLOAD =~ /^((?:.*::)?)(.*)$/) {
    $name =~ /^_default_(.*)/ and croak "method $1 not found";
    my $sub = sub {
      my $method = $_[0]->{m}{$name};
      goto &$method if defined $method;
      my $default = "_default_$name"
      shift->$default(@_);
    }
    no strict 'refs';
    *$AUTOLOAD = $sub;
    goto &$sub;
  }
  croak "bad method name $AUTOLOAD";
}
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and then declare the default methods for your objects as:

sub _default_foo { ... }
sub _default_bar { ... }
...
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