#!/usr/bin/perl

use strict;
use warnings;

$_ = join '', <DATA>;

s/(<p>(?:(?!<\/p>).)*<\/p>)(??{-1 == index $1, "\n\n" and '(*FAIL)'})/-->$1<--/gs;

print;

__DATA__
a 123 456 789
b 123 456 789

c 123 456 789
d 123 456 789

<p>e 123 456 789
f 123 456 789</p>

<p>e 123 456 789
f 123 456 789
g 123 456 789
h 123 456 789</p>

<p>1 123 456 789
2 123 456 789

3 123 456 789
4 123 456 789</p>