from perldoc Getopt::Long

      An incremental option is specified with a plus "+" after
       the option name:

           my $verbose = '';   # option variable with default value (false)
           GetOptions ('verbose+' => \$verbose);

       Using "--verbose" on the command line will increment the
       value of "$verbose". This way the program can keep track
       of how many times the option occurred on the command line.
       For example, each occurrence of "--verbose" could increase
       the verbosity level of the program.