#!/usr/bin/perl

use warnings;
use strict;
use diagnostics;

use IO::Handle;

sub input_number
{
    print "What is your number? (0-25) ";
    STDOUT->flush();
    my $number = <>;
    chomp($number);

    return $number;
}

sub check_number
{
    my $number = shift;

    if ($number< 0) {
        print {*STDERR}
        "The number can not be less than 0, please try again.\n";
        return 0;
    }
    elsif ($number> 25) {
        print {*STDERR}
        "The number can not be more than 25, please enter your number.\n";
        return 0;
    }
    elsif (($number !~ m{\A\d+\z}) || ($number =~ m{\A0\d})) {
        print {*STDERR} "The number must be an integer.";
        return 0;
    }
    else {
        return 1;
    }
}

my $number = input_number();

while (! check_number($number))
{
    $number = input_number();
}

print "You've chosen $number! Congrats.\n";