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$m = ' more gibberish"h" \n URL="http://[10.0.0.3]?id=80943lkjh875kjrv
+f09u548gfpi"\n gibber\n';

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

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

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

$m = ' more gib U berish"h" \n URL="http://[10.0.0.3]?id=80943lkjh875k
+jrvf09u548gfpi"\n gibber\n';

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

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