That bit is just charging up the magic wand. It can just as well be a separate statement:

$| = 1;
print grep --$|, /\b(\w+)\b(?=(.*?)\b\1\b)/g'
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To get even elements instead of odd elements, you just start with the magic wand in the other position:

print grep --$|,($|&&=0)|| /\b(\w+)\b(?=(.*?)\b\1\b)/g'
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or:

$| = 0;
print grep --$|, /\b(\w+)\b(?=(.*?)\b\1\b)/g'
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Why ||= and &&= instead of = is left as an excercise for the reader. Hope that clears things up for you :)