sub set_stock
{
    my( $self, $supplier, $line, $stockLevel ) = @_;

    my $supplierList = ();
    my $supplierNumber = $supplier->supplier_id; 

    # $supplierList will be a reference to a hash of references to hashes.
    unless( $supplierList = $self->get('supplier_list') ) 
    {
        die( "Cannot set stock level when supplier list is corrupt: $supplierList" );
    }

    my $reference = $line->reference;
    $supp->{ $supplierNumber }{ $reference } = $stockLevel;
}