It's just like in regexps 
'XZ' =~ /
   ^
   (?:
      X (?{ print "X" })
      Y (?{ print "Y" })
   |
      X (?{ print "X" })
      Z (?{ print "Z" })
   )
   \z
/x;
print("\n");

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XXZ

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In reply to Re^3: Parse::RecDescent matching same line twice by ikegami
in thread Parse::RecDescent matching same line twice by kudra

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