my $opt = { # this has to be first because the sub-refs
            # below are closures using it.
  grot => [],
};
my $splitter = sub {
    my ($name, $val) = @_;
    push @{$opt->{$name}}, split q{,}, $val;
};
GetOptions( $opt,
  'grot=s@' => $splitter,
) or die "can't parse options";