How to get original request_uri if i use proxy_pass

Hi guys, I need a little help here. 1 of my nginx.conf is as follow :

location ~* ^/help/([A-Za-z0-9-_]+)$ {
proxy_pass http://127.0.0.1:888/help.pl?p1=$1;
}

So for example I open this address : www.mywebsite.com/help/how-to-buy
And on my perl script help.pl I want to get the original request_uri which is help/how-to-buy.
But what i get is help.pl?p1=how-to-buy

So how can I get the original request_uri?