Putting it on one line will illuminate the problem. while ($a < $b) if not $error{$a++}; It thinks error is a hash since immediately following it (and owe to perls ignoring whitespaces) is a opening curly a var and a closing curly.. effectively referencing the $a+1 key of the hash %error.

personally I don't see a better way than

if(!$error)
{
     while($a < $b)
     { 
        $a++;
     }
}
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