my @x = (10, 7, 5, 10, 50, 70, 75, 72, 79, 80);

sub avg2 {
    my @arr = @_; 
    my $sum; 
    foreach my $i (@arr) {
        print "$i: $sum\n"; 
        $sum += $i;
    }
    return $sum/@arr;
}

print avg(@x),"\n";

10:       # undef
7: 10
5: 17
10: 22
50: 32
70: 82
75: 152
72: 227
79: 299
80: 378
45.8

print avg(reverse @x),"\n";

80:       # undef
79: 80
72: 159
75: 231
70: 306
50: 376
10: 426
5: 436
7: 441
10: 448
45.8