From type/Proc/Async

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multi method bind-stdin(IO::Handle:D $handle)
multi method bind-stdin(Proc::Async::Pipe:D $pipe)

Sets a handle (which must be opened) or a Pipe as a source of STDIN. The STDIN of the target process must be writable or X::Proc::Async::BindOrUse will be thrown.

my $p = Proc::Async.new("cat", :in);
my $h = "/etc/profile".IO.open;
$p.bind-stdin($h);
$p.start;

This is equivalent to

cat < /etc/profile

and will print the content of /etc/profile to standard output.