I find it virtually impossible to believe that your spec will never change and you know up front that changing circumstances will never cause you to need to modify your program. The real world simply does not, in my experience, work out that way. Nor do I know anyone else for whom it works that way. (Well I do know of exactly one widely used program which is no longer being modified in any way, shape, or form.)

Therefore if you wish to avoid digging a pit for yourself, I strongly recommend starting with the opposite assumption. Assume that your program will change. Assume further that you don't know how it will change. With these two assumptions you should now see the benefits of trying to modularize the internals of your program as much as possible. Break it up into functions. Make each function do one thing. Make the name of the function say exactly what that function does. Avoid globals. Use strict.pm to remind yourself of globals you might have missed. So on and so forth.

If you do this, then you will be ready to face the inevitable point where reality meets your optimistic hope that you can just write your program and it will really, truly, be done.