$! = "$_ validation failed";

##</code><code>##

=head1 METHODS

=head2 factorial

INPUT : a non-negative integer, n

OUTPUT: the factorial of n

=cut

sub factorial {
    my $number = shift;

    return 1 if $number <= 1;

    return ($number * factorial($number-1)) ;

}

sub validate_non_negative { $_[0] >= 0 }
sub validate_integer      { 1 } # how do you do this?

sub validate {
    my $input = shift;
    my %arg = @_;

    for (@{$arg{as}}) {
        my $func = "validate_$_";
        my $r    =  eval "$func($input)";
        unless ($r) {
            $! = "$_ validation failed";
            return 0;
        }
    }

    warn "validated $input";
    return 1;
}

sub get {
    print "input? ";
    $_[0] = <>;
}

my $input;
get($input)
    and
    (validate($input, as => [qw(non_negative integer)]) or die "error: $!")
    and print factorial($input);