How would you go about inspecting the contents of a scalar to decide if it is a number using only prototypes?
That sounds like more of a job for:

confess("Parameter $n stinks") unless $param[$n] =~ /$goodRule[$n]/;
[download]

Append:
Example of extensive parameter handling:

sub buyWidgets
{
    my $numWidgets = shift;
    my $supplier = shift;
    
    #validate parameters
    unless ($numWidgets =~ /^\d+$/)
    {
        carp("'$numWidgets' is not a valid number of widgets: purchase
+ canceled\n";
        return undef; 
    }
   
    # Validate, and be flexible if possible.   Accept both "ACME" and 
+"hash(0xPtrToACME)", and even ["ACME", "Bob's Widgets", "Black Market
+"]
    if (ref($supplier) eq 'ARRAY')
    {
        my $buyCount = 0;
        foreach (@$supplier)
        {
            my $result = buyWidgets($numWidgets-$buyCount, $_);
            $buyCount += $result if defined($result);
        }
        return $buyCount;
    }
    if (ref($supplier) ne 'Supplier')
    {
        unless (exists $globalSupplierDirectory->{$supplier})
        {
            carp("Cannot buy from '$supplier': not a valid supplier\n"
+);
            return undef;
        }
        $supplier = $globalSupplierDirectory->{$supplier};
    }

    # Do stuff with the good parameters
    return $supplier->buyWidgets($numWidgets);
}
[download]

Untested, and not self-contained.