#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;


my @wordsInOrder;
while (<>) {
    push @wordsInOrder, split /\W+/, lc($_);
}

my (%single, %double, %triple);
my $index = 0;

foreach my $word (@wordsInOrder) {
    $single{$word}++;
    my $next_word = $wordsInOrder[$index+1];
    if($next_word) {
        $double{"$word $next_word"}++;
    } 
    my $next_next_word = $wordsInOrder[$index+2];
    if($next_next_word) {
        $triple{"$word $next_word $next_next_word"}++;
    } 
    $index++;
} 

foreach my $singlet (sort_by_frequency(\%single)) {
    print "$singlet\n";
    foreach my $doublet (sort_by_frequency(\%double)) {
        next unless $doublet =~ /^$singlet\b/;
        print "\t$doublet\n";
        foreach my $triplet (sort_by_frequency(\%triple)) {
            next unless $triplet =~ /^$doublet\b/;
            print "\t\t$triplet\n";
        } 
    } 
} 

sub sort_by_frequency { 
    my $h = shift;
    return sort { $h->{$b} <=> $h->{$a} } keys %$h;
}