Most computer languages use the concept of reserved words, and in this case I think it would simplify your grammar a lot if labels and opcodes were distinct. Here is one way to do it that I adapted from an answer in this mailing list thread (I haven't tried it). 
   opcode: IF | YMOVE | XMOVE | DOWN |
      UP | PRINTLOC | GOTO |
      SET | ADD | SUB | HALT

   label: ...!opcode /[a-z]\w*/i

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In reply to Re: Help with tweaking Parse::RecDescent grammar by tall_man
in thread Help with tweaking Parse::RecDescent grammar by jeffa

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