use Getopt::Long;
    GetOptions(
       'single=s' => \(my $single = ''),
        'multi=s' => \my @multi,
    );

##</code><code>##

    use Getopt::Long;
    GetOptions(
       'single=s' => \(my $single = ''),
        'multi=s' => grep { @$_ = ('a','b') } \my @multi,
    );