"I move it to /etc/httpd/password.txt"

Also consider permissions, who else can get access to this file? I tend to keep this sort of thing in a database.

"What Can I get from it to enhance the code?"

Well it contains a lot of advice which you shouldn't ignore, examples of sane ways to approach such problems in modern frameworks.. Developing secure web applications isn't easy. What you have right now is a splash page redirecting those with a valid username and password to another page, which has no authentication on it whatsoever, nothing to stop anyone emailing the URL to someone else, who would be able to see it without any login details.

CGI::Alternatives suggests other ways to achieve what you're actually trying to do, which will make your life easy if you take the time to learn the basics of what you're doing, rather than trying to implement something where security is "a MUST" without understanding such things. One of those is Mojolicious, which has fantastic documentation, you should pay special attention to the Basics. Tutorial gets you started. Growing goes on to show (wait for it) an example of how to implement an application which requires users to login. Finally HTTPS, for sure, https://letsencrypt.org/ may be of interest.

This requires you to invest time in yourself, learning how to achieve what you want and what the tools available to you do. Modern frameworks make life easier. It's not the mid 1990s anymore (thankfully).

Update: Fixed some typos.