OK, thanks. Considering switching to greedy instead of non-greedy results in the regex

    $string =~ /
        (                                    
            HIT                                
            (?=                                
                [^<]*                        
                (?!                            
                    <a[^>]*>                
                )                
                .*?<\/a>                    
            )                                
            .*?<\/a>                        
        )
    /x;
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but still gives the same results. If the $string

'<a href="#1">--HIT<a name="MUST NOT MATCH">--</a> <a href="#2">2</a>'
[download]

does properly not match and

'<a href="#1">--HIT-<a name="MUST NOT MATCH">--</a> <a href="#2">2</a>
+'
[download]

does "falsely" match I don't understand why the problem lies in the fact that a negative lookahead does not backtrack.