I would review my course material. It is highly unlikely that your teacher asks you something that has not been extensively covered in the lessons and the course material.

Personally, I would look at Stemmers on CPAN, and at perlfunc for the grep and map functions.

So each element of the array is one word? Lingua::EN::StopWords

use strict;
use warnings;
use Lingua::EN::StopWords qw/%StopWords/;
my @words = qw/ .......whatever words you're testing...... /;

@words = grep { !StopWords{$_} } @words;
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All credit to the module's POD. There's also Lingua::StopWords::EN, if the first module doesn't work out for you.

Update: But Corion is probably right. Ask the professor. ;)