You should have no problem with CGI for POST unless you're being given garbage (e.g. being told you are provided something's that "application/x-www-form-urlencoded" when it isn't).

my $url;
if ($cgi->request_method eq 'GET') {
   # Can't use ->param('url') because the client is
   # providing the parameters in a stupid format.
   ( $url = $ENV{QUERY_STRING} ) =~ s/^.*[?&]url=//s;
} else {
   $url = $cgi->param('url');
}
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If you are getting garbage, it can be handled. You've been given the link to the CGI spec. Read it. Your asking questions with no apparent effort at answering them yourself.