# Read each line of file supposing a number per line
#   
while <FILE> {
#
# the += operator will add each number read to 
#the accumulated
#
   $total += $_;
#
# $_ contains the line just read of FILE
# ++ will increment 1 for each line giving a total
# count of lines at the end of the loop
#
   $count++;
}
#
# at the end of the loop you have total = sum of lines
# and count how many lines where read 
#
my $average = $total/$count;
#
# the formula above calculates the average