CGI Version
---------------------

#!/usr/bin/perl -w

use strict;
use warnings;
my ( $totalinterest, $totaldue, $monthlypayment, $round );

use CGI qw/:standard/;
print header, start_html(), start_form, "Principle (amount due): ",
  textfield('principle'), p, "Interest (as a decimal): ", textfield('interest'),
  p, "Duration (in months): ", textfield('duration'), p, submit, end_form, hr;

chomp( my $principle = param('principle') );
chomp( my $interest  = param('interest') );
chomp( my $duration  = param('duration') );

if ( $principle && $interest && $duration ) {

    #   if (param('principle') && param('interest') && param('duration')) {
    print "\n---------------------------<br>";

    $totalinterest = $principle * $interest;
    print "Total interest:   \$$totalinterest<br>";
    $totaldue = $totalinterest + $principle;
    print "Total due:        \$$totaldue<br>";
    $monthlypayment = $totaldue / $duration;

    $round = sprintf "%.2f", $monthlypayment;
    print "Monthly price:    \$$round<br>";
    print "---------------------------<br><br>";

    print "How we did it:<br>";
    print "Actual equation: (principle * interest + principle) / months<br>";
    print "Your equation:   ($principle \* $interest \+ $principle) \/ $duration<br>";
    print "Step one:        ($totalinterest \+ $principle) \/ $duration<br>";
    print "Step two:        ($totaldue) \/ $duration<br>";
    print "Step three:            \$$round";
    hr;
}
else {
    print "Something was missing!<br>";
}

print end_html();