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
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