Works for me on *nix on everything form 5.6 to 5.12 with no errors (after putting something in for the CONSTANT's). Any change you've got some weird unprintable character in it somehow? What are you using for an editor?
The various forms of multiple quoting you have wouldn't work, except the one using a plain doublequote ". That should work, but single quote is also fine. The issue with single quotes acting like package names is when they look like part of the variable; e.g., if you had $x = "$users's coat".