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

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