The reason for your error is because the snippet

sub pr { print main::FO1; }
[download]

is compiled before the the snippet

use constant {
FO1 => "value"
};
[download]

(BTW braces for constant?? Weird.)

so perl has to apply some heuristics while parsing the former code. An all uppercase bareword ("FO1") as a first parameter to print, makes perl assume it's a filehandle.

So what you must do is one of the following:

1. Mark up the print line so it's clear that you don't mean a filehandle. Unfortunately, this:

   sub pr { print +main::FO1; }
   [download]

   still isn't enough.. (Why not? I really expected that this would work.)

   But even if it did work, that probably would still leave you with a prototype mismatch error

2. compile the constant definition for before you use pq;

BTW It's considered best practice to start package (and module) names with an uppercase letter.

Seeing the trouble with the former attempt at a solution, the latter definitely looks like the best option.