【バージョン】
$ perl --version
This is perl, v5.8.0 built for cygwin-multi-64int


【ソース】
use CGI;
$ENV{'REQUEST_METHOD'} = 'GET';
$ENV{'QUERY_STRING'} = 'url=http://www.google.com';
CGI::ReadParse(\%FORM);

print "1 ",$FORM{'url'},"\n";

$FORM{'url'} = 'change';
print "2 ",$FORM{'url'},"\n";

$FORM{'url'} = '';
print "3 ",$FORM{'url'},"\n";



【結果】
$ perl hoge.pl
1 http://www.google.com
2 change
3