s{                              # match and replace on $_
      \b                            # start with a word boundary
        (                           # capture the matches and store in $1
          (?:[a-zA-Z0-9]+_)+        # letters and numbers followed by _ at least once
          [a-zA-Z0-9]+              # must end with letters/numbers
        )
        \(\)                        # then ()
      \b                            # followed by word boundary
     }
     {                          
       <a href="$1.html">$1()</a>   # replace the matched text with this string
     }                              # substituting the value of $1 (the first capture)
     g                              # perform this replace on all matches in $_