#! perl -w
use strict;

package Test;

sub new {
    my $class = $_[0];
    my $self = {'associations' => undef,
	        'balance_by' => undef,
		'comment' => undef};
    
    bless $self, $class;
    return $self;
}

sub balance_by {
    my $self = shift;
    my $entry = shift;
    my %valid_params = map {$_ => 1} ('foo',
				      'bar');

    unless (defined $self->{'balance_by'}) {
	$self->{'balance_by'} = 
	    (defined $entry && $valid_params{$entry})
            ? $entry 
	    : 'foo';
    }
    return $self->{'balance_by'};
}
my $lex = Test->new();
$lex->balance_by('bar');
print $lex->balance_by();