A slightly different approach can give you what you are looking for. By using a lookahead, you can do what you want.

$term = 'Dit is het eerste het is niet het laatste Dit';
@captured = $term =~ /\b(\w\w\w)\b(?=.*\1\b)/g;
print join ' ', @captured;
_____________
Dit het het
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The \b are word boudaries (change from letter/number/underscore) to non-letter/number/underscore or vice-versa.

The (?= looks forward for what comes after it, but remembers where it starts.

The \1 is the same as your \g1 (I unfortunately have an older perl.)

The g at the end means capture them all

het appears twice since it is there three times

Small correction, you missed a word-boundary assertion in the look-ahead. The regex should be:

/\b(\w\w\w)\b(?=.*\b\1\b)/g
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Without the additional \b before \1, three-letter words that are trailing substrings of other words would also match.

Thanks. I missed that copying from the PC that has perl that I tested this on to this one.

Thank you