From type/Proc/Async

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method stderr(Proc::Async:D: :$bin --> Supply:D)

Returns the Supply for the external program's standard error stream. If :bin is passed, the standard error is passed along in binary as Blob, otherwise it is interpreted as UTF-8, decoded, and passed along as Str.

my $proc = Proc::Async.new(:r, 'echo', 'Raku');
$proc.stderr.tap( -> $str {
    say "Got error '$str' from the external program";
});
my $promise = $proc.start;
await $promise;

You must call stderr before you call .start. Otherwise an exception of class X::Proc::Async::TapBeforeSpawn is thrown.

If stderr is not called, the external program's standard error stream is not captured at all.

Note that you cannot call stderr both with and without :bin on the same object; it will throw an exception of type X::Proc::Async::CharsOrBytes if you try.

Use .Supply for merged STDOUT and STDERR.