Apparently, in Lisp, this is referred to as a semi-predicate problem.

The very fact that you are using

    if ($2)
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followed by

    $price = $1 . "." . $2
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shows that you are using $2 as a semi-predicate, i.e. to show validity and to hold value.

Next time, whenever you use if ($var), you should consider if $var needs to be a full-predicate, i.e. strictly a boolean to only show true-or-false.