use strict;
use warnings;

my $string = '<nl><item>asdfasdf</item><item>asdfasdf</item></nl><nl><item>asdfasdf</item><item>asdfasdf</item></nl>';

$string =~ s|<nl>((?:(?!</nl>).)*)</nl>|'<nl>'.&item($1).'</nl>'|egs; 

print $string;

sub item
{
	my ($item) = @_;
	
	my $no =1;
	
	$item =~ s/<item>/'<item num="'.$no++.'">'/egs; 	
	return $item; 
}

output:
-------
<nl><item num="1">asdfasdf</item><item num="2">asdfasdf</item></nl><nl><item num
="1">asdfasdf</item><item num="2">asdfasdf</item></nl>

##</code><code>##

$string =~ s|<nl>((?:(?!</nl>).)*)</nl>|my $item = $1; my $no = 1; $item =~ s/<item>/'<item num="'.$no++.'">'/egs;"<nl>$item</nl>"|egs;