This is suspicious:

foreach my $i ($#$arr)
{
    $avg = ($i + ($avg ? $avg : $i)) / 2;
}
[download]

This only runs once, with $i taking the value of the last index of $arr. You probably meant @$arr.

The next problem is that this isn't an average in the normal sense, it's a weighted average where the last element gets much more weight than the first, more like:

$avg = (...(($arr->[0]/2 + $arr->[1])/2 + $arr->[2])/2 + ... $arr->[$#
+$arr])/2;
[download]

So perhaps you meant a decaying average, but it's not clear from the context. Here's something equivalent, with output:

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
[download]

Making a change to produce the uniformly weighted average:

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
[download]

where the forward and reverse lists produce the same average.

A lot of the syntax would be less cumbersome if you started with my @arr = (...).

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