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

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

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

    foreach my $supp (@$supplierList)
    {
        next unless( $supp->{supplier_id} == $supplierNumber );
        my $reference = $line->reference;
        # Here is the update.
        $supp->{$reference} = $stockLevel; 
    }
}