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

sub avg {
    my @arr = @_; 
    my $avg; 
    foreach my $i (@arr) {
        print "$i: $avg\n"; 
        $avg = ($i + ($avg ? $avg : $i))/2;
    }
    return $avg;
}

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

10:       # undef
7: 10
5: 8.5
10: 6.75
50: 8.375
70: 29.1875
75: 49.59375
72: 62.296875
79: 67.1484375
80: 73.07421875
76.537109375

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

80:       # undef
79: 80
72: 79.5
75: 75.75
70: 75.375
50: 72.6875
10: 61.34375
5: 35.671875
7: 20.3359375
10: 13.66796875
11.833984375