The variables you give after the ? are usually send as %ENV variables. Why not try to print them all and see if your webserver is like that?

#!/usr/local/bin/perl

print "Content-type: text/html\n\n";
print "<pre>\n";

foreach $key (sort keys(%ENV)) {
  print "$key = $ENV{$key}<br/>";
}
print "</pre>\n";
[download]

Now, you seem to want to use CGI, in that case, you get them in param, and you can iterate over them, as described in this previous post:
Best way to parse CGI params

You can then use:

my $q = new CGI;
if ($q->param("URL")) { ... }
[download]

There is also something called Tainted mode, which you should use once your testcode is working, because what If the string after the ? is 2Mb big? or an invalid url etc.

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