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

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

my $from_file = sub {
    my ($name, $val) = @_;
...
    $opt->{$name} = <$fh>;
    close($fh);
};