#! /usr/bin/perl
# vim:ts=4 sw=4 sts=4 et nu fdc=3:
use strict;
use warnings;
use Getopt::Long;

my $x = "unset";

GetOptions(
    'x:s' => sub { $x = $_[1] ? $_[1] : 'true' },
) or exit 255;

print "$x\n";

##</code><code>##

$ perl /tmp/getopt.pl 
unset
$ perl /tmp/getopt.pl -x
true
$ perl /tmp/getopt.pl -x foo
foo