$m = ' more gibberish"h" \n URL="http://[10.0.0.3]?id=80943lkjh875kjrvf09u548gfpi"\n gibber\n';

$u= qr/(:?URL="([^"]*)")?/is; # optional clause (:?xxx)?

if ($m =~ $u) {
    print "<$1><$2>\n";
}else{ print "no match u\n";}

##</code><code>##

[tmp]> perl testResp2.pl 
<><>
[tmp]>

##</code><code>##

$u= qr/.*(?=U)(?:URL="([^"]*)")?/s # moves to just before 'U'

##</code><code>##

$m = ' more gib U berish"h" \n URL="http://[10.0.0.3]?id=80943lkjh875kjrvf09u548gfpi"\n gibber\n';

$u= qr/(?:.*(?=U)(?:URL="([^"]*)")?)*/s; #lookahead to 'U'

if ($m =~ $u) {
    print "<$1>\n";
}else{ print "no match u\n";}

##</code><code>##

$p= qr/RESPONSE\sid="([^"]+?)"
       .*                    #random XML
       disposition="([^"]+?)"
       .*                    #random XML
       (?:.*(?=U)(?:URL="([^"]+?)")?)*
      /sx;