It reads the file to be parsed from the command line following the program invocation. ( perl regex.pl testlines.txt )

It also places the local statement and the code inside a block to limit the changes to $/ to the block. Although this is not necessary for this small program, in a larger program, this will restore $/ to \n after the block is finished for any code that could follow.

#!/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, <>;    
}
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My command prompt:

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.
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