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

$^I = ".bak";  # same as -i option
undef $/;   # slurp whole files!

my $replacement= '\t<?php include(\$_SERVER['DOCUMENT_ROOT'].\"\/includes\/footer.php\"); ?>\n\n';
my $filecontents;
while (defined ($filecontents=<>)) {
   $filecontents =~ s { </div> .*? \K (?=</div>) } { $replacement } x;
   print $filecontents;
   }