#!usr/bin/perl 

use strict; 
use warnings; 

my $input = do{local $/; <DATA>};

$input =~ s|(<p>)|<con>\n$1|;
$input =~ s|(.*</p>)|$1\n</con>|s;

print $input;

__DATA__
<root>
<p>This is para1</p>
<p>This is para2</p>
<date>Wednesday, October 28, 2009</date>
</root>

##</code><code>##

<root>
<con>
<p>This is para1</p>
<p>This is para2</p>
</con>
<date>Wednesday, October 28, 2009</date>
</root>