Good point, since skazat's intent was to build a general container class, where subclasses would only define the desired fields. So how about this (which should also satisfy chromatic :-). No file or global variables. Only define the "fields" subroutine which returns the permitted fields in the desired order. The constructor uses that to build the _permitted element, and the rest of the code is pretty much the same. Defining a new subclass only needs the definition of fields, as shown by SUBPRODUCT below. The fields are not inherited, although that could probably be fixed by traversing the hierarchy calling fields() at each level.

package PRODUCT;
  use Carp; 

  use strict;
  use vars qw($AUTOLOAD);
    
    sub fields {
        return qw(data);
    }

  sub new { 
  my $product = shift; 
  my $class = ref($product) || $product; 
  my $self={};
  bless $self, $class;
  my %fields;
  @fields{$self->fields}=undef;
  $self->{_permitted} = \%fields;
  $self->{$_}=$fields{$_} for (keys %fields);
  return $self;
  }

  sub AUTOLOAD { 

  my $self = shift; 
  my $type = ref($self) 
              or croak "$self is not an object"; 
            
  my $name = $AUTOLOAD; 
     $name =~ s/.*://; #strip fully-qualified portions

  unless (exists $self ->{_permitted} ->{$name} ) { 
  croak "Can't access `$name' field in class $type"; 
  } 


  if (@_) { 
     return $self -> {$name} = shift; 
     }else{
        return $self-> {$name}; 
        
      }  

  }

  sub DESTROY {}

  sub pack {
        my $self=shift;
        my $record=join(':', @{$self}{$self->fields});
        return $record;
  }

package SUBPRODUCT;

use vars qw(@ISA);

@ISA=qw(PRODUCT);

sub fields {
        return qw(name price description);
}

1;
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--ZZamboni