#!/usr/bin/perl

use warnings;
use strict;
use Getopt::Long;

my ($option_test);
GetOptions( "test=s" => \$option_test );

print 'option is not defined.' if ( ! $option_test );

##</code><code>##

$ test.pl
option is not defined.

$ test.pl -t
Option test requires an argument
option is not defined.