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

my $file;
my $match = "that";
my $extra = "Hello, I snook in here\nme too!";
{
   local $/;
   $file = <DATA>;
   $file =~ s/$match\n([^\n]*)\n([^\n]*)/$match\n$1\n$2\n$extra/;
}
print $file;

__DATA__
this
that
leave me
leave me
move me
and me

##</code><code>##

this
that
leave me
leave me
Hello, I snook in here
me too!
move me
and me