Solution: Output and error

Here is a possible solution to the task.

Code

my @data = 3, 5, 7;

my $sum = 0;
for @data -> $n {
    $sum += $n;
    $*ERR.say("running total: $sum");
}

$*OUT.say($sum);

🦋 You can find the source code in the file output-and-error.raku.

On screen, both streams appear — the running totals, then the final result:

running total: 3
running total: 8
running total: 15
15

If standard error is discarded, only the real output remains:

$ raku output-and-error.raku 2>/dev/null
15

$*OUT and $*ERR are the handles for standard output and standard error. Each has its own say method.
Each pass of the loop adds the next number and writes the running total to $*ERR; only the final sum goes to $*OUT. On the screen the progress and the result look alike, but they travel through different streams — which is why redirecting standard error away with 2>/dev/null leaves just the final 15 behind.