Another way to do it ;)

#!/usr/bin/perl 
use strict;
use warnings;

local $/;           
local $_ = <DATA>;
my $c; 
s/(<xxx>)(?{++$c;})/<$c>$1/g;
print;

__DATA__
<xxx>fo</xxx>
<p>ba</p>
<xxx>foo</xxx><xxx>bar</xxx>
<p>foobar</p>
[download]

Output:

<1><xxx>fo</xxx>
<p>ba</p>
<2><xxx>foo</xxx><3><xxx>bar</xxx>
<p>foobar</p>
[download]

HTH,
PooLpi