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

@ARGV == 1 or print "Usage: perl $0 testlines.txt\n" and exit;

{
	local $/ = "\n\n";
	print grep /\b(?:third|four)\b/i, <>;	
}

##</code><code>##

C:\Old_Data\perlp>perl test3.pl testlines.txt
This is line one.
Line Two is this.
Third line starts here.
This is line four.