#!/usr/bin/perl -w
use strict;

# List all non-common words in a block of text

my %stopwords = (a => 1, an => 1, in => 1, the => 1);
my %D2;

while ( <DATA> ) {
   @D2{ map { my $l = lc;
              exists $stopwords{$l} ? () : $l
            } split /\W+/
      } = ();
}

print "$_\n" for sort keys %D2;

__DATA__
This is a demo of an
interesting example
in the post
from PerlMonks.