#!/usr/bin/perl

use strict;
use warnings;

my $abstract = 'Perl is a high-level, general-purpose, interpreted, dynamic programming language.';

my (%double_words, %triple_words, %four_words);

my @words = split(/\s+/, $abstract);

foreach(0..$#words){
   $double_words{$words[$_].' '.$words[$_+1]}= undef if($_<$#words);
   $triple_words{$words[$_].' '.$words[$_+1].' '.$words[$_+2]}= undef if($_<$#words-1);
   $four_words{$words[$_].' '.$words[$_+1].' '.$words[$_+2].' '.$words[$_+3]}= undef if($_<$#words-2);
}

foreach(keys %four_words){ print $_."\n"; } #sample output