#!/usr/bin/perl
use strict;
use warnings;
print '<con>';
my $out;
while(<DATA>){
  $out .= $_;
}
chomp($out);
print "$out</con>\n";
__DATA__
<p>This is param1</p>
<p>This is second para</p>
<p>this is param3</p>
[download]

Do you mean replace the first p-tag in all DATA, or the first p-tag on each line? Ditto for last.

If you did mean by line, step 1 is to drop the 'g' from your regexes.