Q:

Changing $*DISTRO values for testing

I need to test a feature that includes this line:
if $translate-nl && $*DISTRO.is-win

I have tried to reassign a value to $*DISTRO,
$*DISTRO='Windows 10';

but it says:
Cannot modify an immutable Distro (debian (9.stretch))␤

$*DISTRO is a dynamic variable, and it makes sense that it's not modified. That said, is there any other way that code can be tested (other than going to Windows, of course)

A:

my $*DISTRO = ...

Hopefully modifying the original is unnecessary. It's generally unreasonable action-at-a-distance -- almost certainly so if someone has arranged for it to be an immutable value. This is the reason why global variables got such a bad reputation.

A:

To elaborate on raiph's answer: the * in $*DISTRO marks it as a dynamic variable. You can re-declare it any scope, and code called from there will see the redeclared value:
{
    my $*DISTRO = ...;
    # coded called from here sees the redeclared value
}
# code called from here sees the original value

Now, the question remains, what do you put in place of these pesky ...?
In the simplest case, a mock that just has whatever the code under test needs:
{
    my class Distro { has $.is-win }
    my $*DISTRO = Distro.new( :is-win );
    # call your test code here
}

If the code needs more attributes in Distro, just add them to the mock Distro class.
If the code needs a "real* Distro object, for some reason, you can instantiate the built-in one. The constructor .new isn't really documented, but the source code makes it pretty obvious what arguments it expects.

