Rakudo's built-in UTF8 parser will wait for a possible combining character before getc() returns. Using this module, you can read bytes from $*IN and use this module to get unicode characters
use Unicode::UTF8-Parser;
my $stdin = supply { while (my $b = $*IN.read(1)[0]).defined { emit($b) } };
my $utf8 = parse-utf8-bytes($stdin);
$utf8.tap({ say "got $_" });
The module exports the sub parse-utf8-bytes, which take a supply as input and returns a new supply.
If there are Non-UTF8 bytes in the stream, they will be emitted as Int. You have to implement handling of these yourself ($val ~~ Int).