use Validate;
use strict;
use CGI qw(:standard);
	
	my $firstname = Validate->alphanum( param( 'firstname' ) );
	print "Missing or invalid first name\n" unless $firstname;
	
my $stmt = "INSERT INTO tablename VALUES ('$firstname')";
excute...

##</code><code>##

package Validate;

sub alphanum {
    my ($class, $value) = @_;
    return unless $value =~ /^([A-Za-z0-9 -]*)$/;
    return "$1";
}
1;