GetOptions (
  "a=s" => \@a,
  "b=s" => \@b
);
@a = split(/,/,join(',',@a));
@b = split(/,/,join(',',@b));

##</code><code>##

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";

##</code><code>##

my $from_file = sub {
    my ($name, $val) = @_;
    open my $fh, "<$val" or die "can\'t open file \'$val\'";
    local $/ = undef;
    $opt->{$name} = <$fh>;
    close($fh);
};