Well, I assume that you are calling your script mSale.pl, and not .pm. First of all, I strongly descourage you to put your libraries in any place where a web user can get. Said that, suppose you are putting your authenticate.pm in /usr/lib/cgi. Then you'll want to start your script with something such:

#!/usr/bin/perl

use strict ; # ...and...
use warnings ; # always!

# let your cgi find authenticate.pm in /usr/lib/cgi...
use lib '/usr/lib/cgi' ;

# ...and you are safe ;-)
use authenticate.pm

# your CGI goes here :-)
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I hope this helps