use strict;
use warnings;

my @numbers = qw(4 12 18 21 35);

my $average = find_average(\@numbers);
print "The average found for this list is: $average \n" ;

## find_average will cause a fatal error if
## there are no numbers in the array.

sub find_average  
{
   # subroutine gets a sinle value which is a reference
   # to an array, not an arry itself. This is much, much
   # faster if say the array has say 1,000 elements.
   
   my $num_array_ref = shift;
   
   # could be: my ($num_array_ref) = @_;
   # The shift operation is very slightly faster if
   # only one value is involved.
   
   my $sum;   # no need to set $sum=0; 
              # but I wouldn't quibble if you did that.
   
   foreach my $num (@$num_array_ref) 
   {
      $sum += $num;
   }
   
   my $avg = $sum / @$num_array_ref;
   return $avg;
}

__END__
The average found for this list is: 18