I've looked around for a way to easily debug scoping problems. I used App::Trace, and Devel::SimpleTrace, which I prefer to diagnostics. I didn't cleanup the code, but I followed chromatic's advice about single-argument bless being almost always an error and fixed the bless; also, I declared $sound1 and $sound2 in your sound sub.

#!/usr/bin/perl -T

use strict;
use warnings;
use App::Options;
use App::Trace;
use Devel::SimpleTrace;

Animal::Hog->sound();

{
    package Animal::Hog;

    sub new {
        &App::sub_entry if ($App::Trace);
        my ($this, @args) = @_;
        my $class = ref($this) || $this;
        my $self = { @args };  
        bless $self, $class; 
        &App::sub_exit($self) if ($App::Trace);
        return($self);
    }

    sub sound {   
        my $sound1 = 'knor1';
        my $sound2 = 'knor2'; 
        &App::sub_entry if ($App::Trace); 
        my $self = shift;
        &App::sub_exit($_) if ($App::Trace);
        print $sound1, "\n",
              $sound2, "\n";
        }
        if ($App::debug && &App::in_debug_scope) {
            print "Debug output\n";
    }
}
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