package Counter;
use strict;
use Carp;

sub new {
    my ($class,$value) = @_;
    $value ||= 0;
    my $self = bless {}, $class;
    croak "Argument to constructor must be positive integer."
    	if ! $self->_good_value( $value );
    $self->{_value} = $value;
    return $self;
}

sub value {
    my $self = shift;
    $self->{_value} = $_[0] if $self->_good_value($_[0]);
    return $self->{_value};
}

sub increment_value {
    my $self = shift;
    $self->{_value}++;
}

sub decrement_value {
    my $self = shift;
    $self->{_value}--;
}

sub _good_value {
    my ( $self, $value ) = @_;
    return 0 if ! defined $value;
    $value =~ /^\d+$/;
}

1;

##</code><code>##

#!/usr/bin/perl -w
use strict;

BEGIN {
    unshift @INC, '.'; # add current directory to include path
    use Counter;
}

my $counter = Counter->new;

print $counter->value, "\n";
$counter->increment_value;
print $counter->value, "\n";
$counter->decrement_value;
print $counter->value, "\n";
$counter->value(3);
print $counter->value, "\n";