#! /usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

my $done;
while (! $done) {
    print << '__MENU__';
Main Menu
  1. Get date and time
  2. Go to submenu
  3. Quit
__MENU__

    chomp( my $reply = <> );
    next unless $reply =~ /^[123]$/;

    if ($reply == 1) {
        say scalar localtime;

    } elsif ($reply == 2) {
        $done = submenu();

    } else {
        $done = 1;
    }

}

sub submenu {
    my $done;
    while (! $done) {
        print << '__MENU__';
Submenu
  1. Get username
  2. Back
  3. Quit
__MENU__

        chomp( my $reply = <> );
        next unless $reply =~ /^[123]$/;

        if ($reply == 1) {
            say scalar getpwuid $>;

        } else {
            $done = $reply;
        }
    }
    return $done == 3
}