I tend to comment regexps like this:

$path=~/^\s*([\w-]+)  # elt
                 \s*\[\s*\@      #    [@
                 ([\w-]+)        #      att                     $1
                 \s*=\s*(["'])   #         = "          (or ',) $2
                 (.*?)           #            value             $3
                 \3\]\s*$/gx)    #                 "]   (or ')
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It works very well for moderately complex regexps.