You have DECLARED your subroutines, but not CALLEd them.

The code works if you add the calls around the "$average" caclulation, like:

div(); # CALL the sub named div

$average=($g1 + $g2 + $g3 + $g4 + $g5 + $g6) / $div;

letter(); #CALL the sub named letter
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Of course, as others have pointed out, there is much room for code and coding style improvement.

Update:Here is a more concise, and idomatic version:

#!/usr/bin/perl
use strict;
use CGI qw|:standard|;

my ($count,$total);
my @gradeletter = qw| A A- B+ B B- C+ C C- D+ D |; # no F because that
+ is default;
my @gradeval = qw|92.5 89.5 87.5 82.5 79.5 76.5 72.5 69.5 66.5 62.5 |;
   
for(1..6){
  my $value = param ("g$_");
  $count++ if $value > 0;
  $total += $value;
}
$count ||=1; # Avoid divide by zero
my $average = $total / $count;
my $letter = "F"; # Default , unless a better one is found

for (0..$#gradeletter){
  if ($average > $gradeval[$_]){
     $letter = $gradeletter[$_];
     last;
  }
}

$letter = "INVALID" if $average > 100;


print ul(li(" The letter grade is $letter" )), "\n";
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