Very cool, but the output of print $2, $/ while /(\b\w+\b)(?=(.*?)\1)/g; is:

 brown fox jumped over the other
 fox jumped over the other quick
 jumped over the other quick brown
 o  #????
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In order to pass over the "o" (from "the o_the_r") the backreference in the lookahead needs to be anchored to word boundaries, ie:

print $2, $/ while /(\b\w+\b)(?=(.*?)\b\1\b)/g;

Not to mention, what is the desired output if the string is "the one the two the"?