sub average_over_time {

#
# $ages[x,0] = time of article arrival
# $ages[x,1] = age of article;
# articles are thrown out of the pipe if they've arrived more than $maxage
# ago.
#
    my (@ages,$sumages);
    
    my $maxage=shift @_;

    return sub {
        my $limit=time-$maxage;

# Squeeze out events that have timed out
        while (($#ages>-1)              
               && ($ages[0][0]<$limit)) { 
            $sumages-=$ages[0][1];
            shift @ages;
        }

        if (defined($_[0])) {
            push(@ages,[time,$_[0]]); $sumages+=$_[0];
        }
        $sumages=0 if $#ages == -1;
        return ($#ages>-1)?int($sumages/($#ages+1)):0;
    }
}
<code>

That gives me the ability to say:<code>
my $mail5=average_over_time(300);
my $mail60=average_over_time(3600);

##</code><code>##

$db{"time-mail5"}=&$mail5($time);
$db{"time-mail60"}=&$mail60($time);