compare 2 mortgage options

mortgage_example.pl

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

my $n          = 21 * 12;    # remaining amortization in number of months
my $ingblended = 3.84;
my $ingbroken  = 3.08;
my $tdrate     = 2.79;

my $outside_penalty = 15215.42;
my $inside_penalty  = 11012.97;

my $INGblended = 402403.98 - 2558.83;
my $INGbroken  = $INGblended + $inside_penalty;
my $TDP        = $INGblended + $outside_penalty;

sub rate {
    my $rate = shift;
    return $rate / 100.0 / 12.0;
}

sub payment {

    # http://en.wikipedia.org/wiki/Mortgage_loan#Capital_and_interest
    my ( $P, $r, $n ) = @_;
    $r = rate($r);
    my $e = ( 1 + $r )**$n;
    return ( $P * $r * $e ) / ( $e - 1 );
}

sub monthly_payment {

    # http://en.wikipedia.org/wiki/Fixed_rate_mortgage
    my ( $P, $r, $n ) = @_;
    $r = rate($r);
    my $e = 1.0 / ( ( 1 + $r )**$n );
    return $r * $P / ( 1 - $e );
}

say sprintf "ING payment (blended): %.2f",
  payment( $INGblended, $ingblended, $n );
say sprintf "ING payment (broken) : %.2f",
  payment( $INGbroken, $ingbroken, $n );
say sprintf "TD  payment          : %.2f", payment( $TDP, $tdrate, $n );