Or even:

package Client;

sub CreateInvoiceCalculator {
    my $self = shift;

    return (qw/DefaultClientIC
               UnusualClientIC
               VeryStrangeClientIC
               MartianClientIC/[$self->{client_type}]) -> new;
}

# ...

my $ic = $client->CreateInvoiceCalculator;
[download]

You probably cannot totally avoid some sort of "client-type to client IC class" relation so let it be hidden inside Client class which is probably the only to need knowledge of multiple ways of invoice calculation (read: InvoiceCalculator descendants).

Or.. you can parallel your InvoiceCalculator hierarchy with a similar Client hierarchy where each Client descendant will use its own calculator. That's another story altogether.

Read "Design Patterns", really. It's full of wisdom.