use strict;
use warnings;
use Data::Dump::Streamer;

use constant ORDER => 3;

my %chains;

while (<DATA>) {
    my @words = split;
    my @chain;
    
    for (@words) {
        push @chain, $_;
        next if @chain < ORDER;
        shift @chain if @chain > ORDER;
        
        my $root;
        
        for my $index (0 .. ORDER - 1) {
            if (defined $root) {
                ++$root->[1]{$chain[$index]}[0];
                $root = $root->[1]{$chain[$index]};
            } else {
                ++$chains{$chain[$index]}[0];
                $root = $chains{$chain[$index]};
            }
        }
    }
}

Dump (\%chains);


__DATA__
I will not eat them in a bar.
I will not eat them in a car.
I will not eat them Sam I am.
I will not eat green eggs and ham!