I might have simplified this too much, but:

$html = '<!--% xxxx UNO xxxx %-->   <!--% xxxx DOS xxxx %-->';
$html =~ s/<!--%(.*?) DOS (.*?)%-->/GONE/s;
print $html;

what I want to produce is leave the first comment tag, while
replacing the second with GONE (and yes I do need to grab the xxxx text)

the 'xxxx' in both tags may be anything, and may span several lines
I may not always be looking for 'DOS', this was just an example.

my problem is that it chews up the first one, starting with
<!--% from UNO's tag, and ending with --> from DOS's tag..