package Processor;

# constructor in base class only

sub new {
  my $pkg = shift;
  bless {}, $pkg;
}

# init, process, finish in subclasses

1;

##</code><code>##

package Processor::SomeSubClass;

@ISA = ( Processor );

sub init {
  my $self = shift;
  # do something
}

# should be called process_line
# this one upcases it
sub process {
  my $self = shift;
  my $line = shift;
  return uc( $line );
}

sub finish {
  my $self = shift;
  # do something
}

1;

##</code><code>##

#!/usr/bin/perl

use Processor;
use Processor::SomeSubClass;

my $p = Processor::SomeSubClass->new();

$p->init;
while (<>) {
  print $p->process( $_ );
}
$p->finish;