That's better written using exists() and undef values for %dups. You've written more code than you had to. There's something to be said for only writing the necessary bits.

my @kewords = ...;
my %dups;
for my $keyword (@keywords) {
    if (exists $dups{$keyword}) {
        # dup
    }
    else {
        $dups{$keyword} = undef;
    }
}
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