package Length;

sub new {
    my ($class, $value, $unit) = @_;
    # Create $self and store the value in standard form. 
    # In case of length this means converting the length to meters.
}

sub as_km  { shift->{_m} / 1000 }
sub as_hm  { shift->{_m} / 100 }
sub as_dam { shift->{_m} / 10 }
sub as_m   { shift->{_m} }
sub as_dm  { shift->{_m} * 10 }
sub as_cm  { shift->{_m} * 100 }
sub as_mm  { shift->{_m} * 1000 }

# Or, if this were really something I made, just:
sub as {
    my ($self, $unit) = @_;
    # ...
}