G'day Ea,

I see ++Corion has given plenty of information on why you can't assign to a constant.

Constants, such as the PI you have in your code, are subroutines with a prototype of '()' which allows them to be inlined (see "perlsub: Constant Functions"); as such, they can be redefined.

I really don't recommend that you use the following in any production code but, purely as an academic exercise, consider this:

#!/usr/bin/env perl

use strict;
use warnings;

use constant PI => 3.14;

eval {
    no warnings 'redefine';
    sub PI () { 3 }
};

warn "You can't round down PI to 3" if $@;

print 'PI now set to ', PI, "\n";
[download]

When run, this outputs:

PI now set to 3
[download]

No errors or warnings are emitted.