my %stopwords;
@stopwords{(qw(a i at be do to or is not no the 
               that they then these them who where 
               why can find on an of and it by))} = 1 x 27;


##</code><code>##

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

##</code><code>##

use Set::Scalar;
$stopwords = Set::Scalar->new;
$stopwords->insert( qw(a i at be do to or is not no the 
               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";

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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

my $not_in = $s - $members;
my $in     = $s->intersect($members);

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


##</code><code>##

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