my @cat = qw( cat );
run \@cat,
        '<pipe', \*PIPE,
        '>', new_chunker("\n"), sub {
                my $line = shift;
                print $line;
                push @out, $line;
        }, '2>', new_chunker("\n"), sub {
                my $line = shift;
                print STDERR $line;
                push @err, $line;
        } or die $?;
print PIPE "some input\n";
close PIPE;
print "out @out";
print "err @err";