If you make the following changes, it should print "4": 
package Average; # D e c l a r e  a  p a c k a g e
                # A v e r a ge  a  l i s t  o f  g r a d e s
sub av {
$total = 0;
@grades = (3,4,5);

$num_of_grades =$#grades + 1;
foreach $grade ( @grades ) {
     $total += $grade;
}
 $res = $total/$num_of_grades;   #  W h a t  g e t s  r e t u r n e d
#print $res;
return $res;
}

1;
#&av;

[download]

...and your script... 

use warnings;
use Average;

$res = Average::av();
print $res;

[download]

You really meant to return a value ($res) from your sub. Eventually, you will probably also want to pass something as an argument to your sub. See also:

In reply to Re: Finding average using package subroutine by toolic
in thread Finding average using package subroutine by pinnacle

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