$ perl -MO=Deparse,-p -e 'fun(-url => $httpRefer);'
fun((-'url'), $httpRefer);
-e syntax OK

$ perl -MO=Deparse,-p -e 'fun(url => $httpRefer);'
fun('url', $httpRefer);
-e syntax OK

##</code><code>##

$ perl -MO=Deparse,-p -e 'fun(-url);'
fun((-'url'));
-e syntax OK

##</code><code>##

$ perl -Mstrict -Mwarnings -le '
    sub fun { print "@_" }
    fun(1, url);
    fun(2, -url);
    fun(3, -url, "httpRefer");
    fun(4, -url => "httpRefer");
    fun(5, url, "httpRefer");
    fun(6, url => "httpRefer");
'
Bareword "url" not allowed while "strict subs" in use at -e line 3.
Bareword "url"¬ allowed while "strict subs" in use at -e line 7.
Execution of -e aborted due to compilation errors.

##</code><code>##

$ perl -Mstrict -Mwarnings -le '
    sub fun { print "@_" }
    #fun(1, url);
    fun(2, -url);
    fun(3, -url, "httpRefer");
    fun(4, -url => "httpRefer");
    #fun(5, url, "httpRefer");
    fun(6, url => "httpRefer");
'
2 -url
3 -url httpRefer
4 -url httpRefer
6 url httpRefer