# Regular CGI
my $quantity = $q->param('quantity');            # Tainted

# "Fancy" CGI of your own construction
my $quantity = $q->SafeNumberParam('quantity');  # De-tainted

##</code><code>##

sub SafeNumberParam
{
    my ($self)  = shift;
    my ($param) = @_;

    my ($number) = $self->param($param) =~ /^(\d+)/;

    return $number;
}