*** 2007-08-17 08:30:51 *** [UID] = 030318520070817083011                                       

   followed by information, including line breaks and empty lines, repeat of the same 21-digit number, etc.
*** 2007-08-17 09:25:17 *** [UID] = 030104220070817092407                                          
Reason = Web server error
*** XML Request ***
more data, line breaks, spaces, etc.

##</code><code>##

$badxml = `cat /mmf/log/BadXMLXact*`;
$badxml =~  /@ARGV[0](.|\n)+/;
$& =~ m/\*\*\*.*UID/;

$output =  $`;
for ($output )
                {
s/\s{3,}/\n/g;
                }
print $output;

##</code><code>##

perl -00 -nle 'print if /21-digit-number(.|\n)+(\*\*\*\.*UID)/'