For debugging, set $::RD_TRACE = 1. It is very verbose, but allows you to debug your grammars without risk of altering their behavior. For your script, it outputs this: (truncated by me)

    Parse::RecDescent: Treating "/\w/" as a /../ pattern terminal
printing code (9723) to RD_TRACE
 1|  start   |Trying rule: [start]                  |
 1|  start   |                                      |"ABCD"
 1|  start   |Trying production: [character         |
  |          |character character]                  |
 1|  start   |Trying subrule: [character]           |
 2|character |Trying rule: [character]              |
 2|character |Trying production: [/\w/]             |
 2|character |Trying terminal: [/\w/]               |
 2|character |>>Matched terminal<< (return value:   |
  |          |[A])                                  |
 2|character |                                      |"BCD"
 2|character |>>Matched production: [/\w/]<<        |
 2|character |>>Matched rule<< (return value: [A])  |
 2|character |(consumed: [A])                       |
 1|  start   |>>Matched subrule: [character]<<      |
  |          |(return value: [A]                    |
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