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