#!/usr/bin/perl
use strict;
use warnings;

my @lines = split /\n/, <<EOF;
I need your help.
I have an array containing sentences.
I want to order the sentences according to a given word
I am a nice boy
You are a nice boy
They are not good at all 
EOF

print "$_\n" for order_line(@lines);

sub order_line {
	return sort by_3d_word_from_right @_;
}

sub by_3d_word_from_right {
	{ lc((split ' ', $a)[-3]) cmp lc((split ' ', $b)[-3]) }
}