#!/usr/bin/perl

our $DecayParam= 0.5;
our $NoChoice= "nan";


  sub computeavg {
    my $sum=0; my $sumn=0;
    my $decayed=1.0;
    my @list= reverse(@_);
    # check decaying. not sure if this is correct.
    foreach my $v (@list) { if ($v ne $NoChoice) { $sumn+= $decayed; $sum+=$v * $decayed; $decayed*= $DecayParam; } }
    return ($sumn>=1) ? ($sum/$sumn) : $NoChoice;
  }

my $v= computeavg( 1,2,3,4,5 );
print "value is $v\n";