I don't recommend writing applications that interact with a user, asking the user to enter data. Such applications may seem beneficial to the end user, but not if that end user is another application. Rather, use tools that provide a means to accomplish both readability (for humans) and non-inter-activeness (for non-humans). In other words, consider using command line arguments over reading in "raw" STDIN.

#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dumper;
use Getopt::Long;
use Pod::Usage;

my @opts = qw(
    starting-amount
    current-age
    retire-age
    deposit
    interest
    expected
);

GetOptions (
    ( \my %args => map "$_=i", @opts ),
    'help' => \my $help,
    'man'  => \my $man,
);
pod2usage( -verbose => 1 ) if $help;
pod2usage( -verbose => 2 ) if $man;

for (@opts) {
    pod2usage(
        -msg     => "$_ is required",
        -exitval => -1,
        -verbose => 0,
    ) unless defined $args{$_};
}

pod2usage(
    -msg     => "retire-age must be greater than current-age",
    -exitval => -1,
    -verbose => 0,
) if $args{'retire-age'} <= $args{'current-age'};


print Dumper \%args;

__END__
 
=head1 NAME

retirement-calculator - app to calculate retirement amount

=head1 SYNOPSIS

retirement-calculator [options]

 Options:
   --starting-amount   the amount to start retirement with
   --current-age       how old you currently are
   --retire-age        what age you want to retire at
   --deposit           amount deposited per year
   --interest          annual interest rate
   --expected          what you expect to retire with
   --help              list usage
   --man               print man page

=cut
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)