use warnings;
use strict;
use feature 'say';

my $limit_percent = 0.15;

my @nums = ((75) x 5, 99, (75) x 4); # 10 numbers

filtered(\@nums);

for (qw(75 74 75 90 74 75 7)) {
    # shift off oldest number, push the next new one

    shift @nums;
    push @nums, $_;

    say "Added: $_";

    filtered(\@nums);
}

sub filtered {
    my ($nums) = @_;

    my $sum;
    $sum += $_ for @$nums;

    my $avg = $sum / scalar @$nums;
    my $range_limit = $avg * $limit_percent;

    my @filtered_nums;

    for (@nums) {
        if ($_ > $avg + $range_limit || $_ < $avg - $range_limit) {
            say "Removing: $_";
            next;
        }

        push @filtered_nums, $_;   
    }

    my $filtered_avg;
    $filtered_avg += $_ for @filtered_nums;

    return if ! scalar @filtered_nums;

    $filtered_avg = $filtered_avg / scalar @filtered_nums;

    printf( 
        "Filtered %.2f, Avg %.2f, Limit: %.2f\n\n", 
        $filtered_avg, 
        $avg, 
        $range_limit
    );
}

##</code><code>##

Removing: 99
Filtered 75.00, Avg 77.40, Limit: 11.61

Added: 75
Removing: 99
Filtered 75.00, Avg 77.40, Limit: 11.61

Added: 74
Removing: 99
Filtered 74.89, Avg 77.30, Limit: 11.59

Added: 75
Removing: 99
Filtered 74.89, Avg 77.30, Limit: 11.59

Added: 90
Removing: 99
Filtered 76.56, Avg 78.80, Limit: 11.82

Added: 74
Removing: 99
Filtered 76.44, Avg 78.70, Limit: 11.80

Added: 75
Removing: 90
Filtered 74.78, Avg 76.30, Limit: 11.44

Added: 7
Removing: 90
Removing: 7
Filtered 74.75, Avg 69.50, Limit: 10.42