Solution: A dynamic variable

Here is a possible solution to the task.

Code

my $*user = 'guest';

sub whoami {
    say "running as $*user";
}

whoami();

{
    my $*user = 'admin';
    whoami();
}

🦋 You can find the source code in the file a-dynamic-variable.raku.

running as guest
running as admin

The * twigil makes $*user dynamic. whoami never takes it as a parameter — it finds the value by looking outward through the call stack, so the first call reports the default guest.
The inner block redeclares $*user as admin for the duration of that block. The same whoami now sees admin, because dynamic lookup follows whoever is currently on the stack. Overriding a dynamic variable in a scope is how you grant elevated context to everything called from it — without changing whoami at all.