I think you need to take a step back from the problem, and reconsider how you want to represent this data.

You might retrieve multiple answers to a question from your online form and place them into a string next to each other using the join command so that selecting both A and B might give you $form[5] = "AB".

You would then loop through the questions checking the length of this string, which is going to be greater than 1 if multiple choices have been selected. Use a regular expression to see if the correct choice is one of those included in the string, with something like

$ok = $form[$i] =~ /$ans[$i]/;  # e.g. $ans[5] is "A"
$points += 0.5 if $ok;
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There are other ways to do it of course, for example you could use the index command which returns -1 if it doesn't find the substring for which you are looking. You could also keep the form data in a hash, or request the multiple values to be returned into an array. See the documentation for CGI.pm as described in this node for more information about that.