use CGI ':standard';
use warnings;
use strict;

my $total = 0;
my $div = 0;
for (1 .. 6) {
   my $g = param('g' . $_);
   if ($g ne '') {
      $total += $g;
      $div++;
   }
}

die "Error: div by 0" if $div == 0;

my $average = $total/$div;