Command Line Version
-----------------------

#!/usr/bin/perl -w

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

print "Principle:";
chomp( my $principle = <STDIN> );

while ( $principle !~ /^-?\.?\d+(?:\.\d+)?$/ ) {
    print "Number you supplied did not look like a real number.\n";
    print "Only numbers and decimals are allowed, no commas!\n\n";
    print "Principle:";
    chomp( $principle = <STDIN> );
}

print "Interest (as a decimal):";
chomp( my $interest = <STDIN> );

while ( $interest !~ /^-?\.?\d+(?:\.\d+)?$/ ) {
    print "Number you supplied did not look like a real number.\n";
    print "Only numbers and decimals are allowed, no commas!\n\n";
    print "Interest (as a decimal):";
    chomp( $interest = <STDIN> );
}

print "Duation (in months):";
chomp( my $duration = <STDIN> );

while ( $duration !~ /^-?\.?\d+(?:\.\d+)?$/ ) {
    print "Number you supplied did not look like a real number.\n";
    print "Only numbers and decimals are allowed, no commas!\n\n";
    print "Interest (as a decimal):";
    chomp( $duration = <STDIN> );
}

print "\n---------------------------\n";

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

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

print "How we did it:\n";
print "Actual equation: (principle * interest + principle) / months\n";
print "Your equation:   ($principle \* $interest \+ $principle) \/ months\n";
print "Step one:        ($totalinterest \+ $principle) \/ months\n";
print "Step two:        ($totaldue) \/ months\n";
print "Step three:            $round";