merlyn has the answers, as always. But just to make things a bit more easier for you, the simple way to get started on your transition from cgi-lib.pl to CGI.pm, is to use the compatibility mode. For instance:

use CGI;
CGI::ReadParse(*in);
[download]

Voilá! You have your good ol' %in...

#!/home/bbq/bin/perl
# Trust no1!