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my %stopwords;
...
               that they then these them who where 
               why can find on an of and it by))} = 1 x 27;

my @salient_word = grep { not $stopwords{$_} } @word ;

use Set::Scalar;
$stopwords = Set::Scalar->new;
...
               that they then these them who where 
               why can find on an of and it by));
@word = split /\s+/, "Oh say can you see by the dawn's early light";

my @salient_word = grep { not $stopwords->has($_) } @word ;

$word = Set::Scalar->new(@word);
my $salient_word = $word - $stopwords;
# or this:
my $salient_word = $word->difference($stopwords);

$members = Set::Scalar->new(@members);

...
$s->delete($in->members);
$s->insert($not_in->members);

 $N = ($a - $b); 
 $N->insert($b - $a);