[perl #132242] [BUG] Proc::Async running with yes command returns superfluous output and hangs

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# New Ticket Created by  Itsuki Toyota 
# Please include the string:  [perl #132242]
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See the following result:

$ perl6 -e 'my $proc = Proc::Async.new("yes"); $proc.stdout.head(1).tap(&say); 
await $proc.start;'

y
y
y
y
y
y
y
y
...


I think tap(&say) should output just a 'y' char and completes its work.


$ perl6 --version
This is Rakudo version 2017.09-254-gde0533c4d built on MoarVM version 
2017.09.1-575-gd4e230a69
implementing Perl 6.c.