Actually I like /x, mostly actually to comment properly which parts of the regexp I want to capture:

$exp=~ m{^([\w]+|\*)\s*   # a name or '*'      ($1)
          (?:             # an optional condition in brackets
           \[\s*(\w+)     #   name             ($2)
           \s*=\s*        #        =
           "([^"]*)"      #           "string" ($3)
           \s*\])?}x;
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I really find it much easier to maintain later if I know where in the regexp I capture a specific value.