Why did you ignore the hint to avoid <pre>-Tags around your post?

Did you only think about the given code, or did you try it?

You also could have a look at Getopt::Long, which explains what happens, if an option doesn't have its optional argument...

Another possibility is something like this:

#! /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";
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It's up to you to refine the checks and values to fit your needs.

added:

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