You guided towards the solution. The solution:

 $form->field(
        name       => 'answers',
        label      => 'Answers:',
        type       => 'checkbox',
        multiple   => 1,
        options    => [['value','displayed1'],['value2','displayed2']]
+,
        add_after_option => "<br>",
        linebreaks => 1,
        cleanopts => 0,
        validate => \&sub_call
    );
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See options solves 1.

validate => \&sub_call solves 2.

cleanopts solve 3.

See options solves 1.