<root>
<id>274660P4</id>
<p>This is para1</p>
<p>This is para2</p>
<date>Wednesday, October 28, 2009</date>
</root>
[download]

<root>
<id>274660P4</id>
<con><p>This is para1</p>
<p>This is para2</p>
<p>This is para3</p>
<p>This is para4</p>
<p>This is para5</p><con>
<DD>Wednesday, October 28, 2009</DD>
</root>
[download]

Wild stab in the dark.

#!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>
[download]

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

Any other way of using substitution other than my $input = do{local $/; <DATA>}; I am reading the file at the beginning and some code like splitting the file into many files.

#!/usr/bin/perl

my $tag;
my $output;
my $fh;

while (<DATA>) {
    chomp;
        if(/^\.\.(.*):$/)     # match line
                {

                $fh = sub_output($output, $tag, $fh);
                $output = "";
                $tag = $1;
                 print $tag;
                }
                 else
                {            # not a {TAG} line
                next unless($tag);
                next if(/^\s*$/);
                $output .= ($output) ? " $_" : "<$tag>$_";
                }
        } # End of While Loop


$fh = sub_output($output, $tag, $fh);

if($fh) {
    print $fh "</root>\n";
    close($fh);
}
exit(0);
# Subroutine to open the file with the filename as DN
sub sub_output {
    my ($output, $tag, $fh) = @_;
    if($output) {
        if($output =~ m/<DN>(.*)/) {
            if($fh) {
                print $fh "</root>\n";
                close($fh);
            }
            open($fh, '>', "$1.xml") or die "$1.xml: $!";
            print $fh "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n";
        }

                $text = "$output</$tag>";

                $text =~ s/<DN>.*<\/DN>//;
                # Here i should substitute <con> for first <p> tag and
+ <p></con>

               print $fh "$text>\n";
                 }
    return($fh);
  } # End of sub sroutine

__DATA__
..DN:
1
..id:
000044119
..DD:
Friday, October 30, 2009
..p:
THis is param1
..p:
THis is param2
..p:
THis is param3
..DN:
2
..id:
000044119
..DD:
Friday, October 30, 2009
..p:
THis is param1
..p:
THis is param2
..p:
THis is param3
[download]