Make msm a flag...

#!/usr/bin/perl

use strict;
use warnings;
use Getopt::Long; 
 
my $string = 'string';
my $length = 'length';
my $msm    = 'msm';

my $result = GetOptions ('des=s' => \$string,
                         'a=s'   => \$string,
                         'd=s'   => \$string,
                         'r=i'   => \$length, 
                         'v=s'   => \$string,
                         'msm'   => \$msm);
[download]

Or simply my $msm;