Extremely simple pension modeller

model.pl

#!/usr/bin/env perl
#  -*- mode: cperl -*-
#
# ./model.pl 0.26 0.02 40 3 0.5 25
#
# 0.26 = fraction of salary contribution total (employer+employee)
# 0.02 = fractional expected annual asset growth
# 40   = number of years working
# 3    = salary multiple for lump sum payment
# 0.5  = fraction of salary paid out in retirement
# 25   = number of years retirement

use strict;
use warnings;

my ($contrib, $growth, $years_work, $lump_multiple, $payout, $years_retired) = @ARGV;

my $pot = 0;
my $salary = 100;
printf "annual salary is %d\n", $salary;

foreach (1 .. $years_work) {
  $pot += $salary*$contrib;
  $pot *= (1+$growth);
}

printf "pot is %d after %d years contributions at %d%% of salary and %d%% annual growth of pot\n",
  $pot, $years_work, $contrib*100, $growth*100;

my $lump_sum = $salary*$lump_multiple;
$pot -= $lump_sum;

printf "lump sum payout of %d upon retirement\n", $lump_sum;

foreach (1 .. $years_retired) {
  $pot -= $salary*$payout;
  $pot *= (1+$growth);
}

printf "after %d years retirement paying %d%% of salary, with %d%% growth of (shrinking) pot, the balance is %d\n",
  $years_retired, $payout*100, $growth*100, $pot;

Please see the new version here:

https://gist.github.com/bobular/ffd25244538db9daa92381b1efe8645c
This model does not take into account career progression (annual salary increments) and I have also made some more minor changes to make the model even more conservative.