#!/usr/bin/perl

use strict;
use warnings;
use Set::Scalar;

# use Data::Dumper;

my @str1= qw(Poet Blake had a milky white cat He used to call it Pussy
+);
my @str2 = qw(Poet Blake had a white cat and used to call it Pussy);

my $s1= Set::Scalar->new(@str1);
my $s2 = Set::Scalar->new(@str2);

my $result = $s1*$s2;

print $result;

__END__

(Blake Poet Pussy a call cat had it to used white)

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See Set::Scalar

Perhaps this helps. Best regards, Karl

Update: You wrote you are a teacher. So i think you can do the math by yourself ;-)

Regards, Karl, Earl of GuttenPlag.

P.S.: And diff might be a weapon too...