#!/usr/bin/perl
use strict;
use warnings;
my $flag=0;
while(<DATA>){
 if($flag == 0){
                if($_ =~ m/<p>/){
                $_ =~ s/<p>/<con>/;
                $flag=1;
                }
                }
print $_;

}
__DATA__
<root>
<p>This is para1</p>
<p>This is para2</p>
</root>
[download]

The output should be

<root
<con>
<p>This is para1</p>
<p>This is para2</p>
</con>
</root>
[download]

Only after the last p tag in the file I have to add </con>