package YourExtension;
use base 'TheOriginal';

sub new {
    my $class = shift;
    my $extended = $class->SUPER::new( @_ );
    my %self = (
        CELLULAR => undef,
        EMAIL => undef,
    );
    my $closure = sub {
        my $field = shift;
        if ( exists $self{$field} ) {
            if ( @_ ) { $self{$field} = shift }
            return $self{$field};
        }
        else {
            return $extended->$field( @_ );
        }
    };

    bless( $closure, $class );
    return $closure;
}