Seems to that you need an Invoice-type field in your table. Then when you instantiate an instance of your InvoiceCalculator passing the Client object, it (the IC constructor) can query the invoice-type from the passed instance and bless the instance into the correct subclass. Concatenating the Invoice-type (say 'A', 'B', 'C' etc.) with it's classname to form the subclass name to bless into.

package InvoiceCalculator;
sub new{
    my( $class, $client ) = @_;    
    my $InvoiceType = $client->InvoiceType;
    my $self = ....;
    return bless $self, "$class::$InvoiceType";
}
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This way, adding new invoice type just requires you to create the appropriate subclass of InvoiceCalculator, and changing the type of a clients invoice just requires updating the appropriate field in that clients record in the table. You get complete flexibility and avoid hardcoded if/else cascades, and minimise code changes.