$ strace -etrace=select,read ./754685.pl >/dev/null
...
select(16, [6 9], NULL, NULL, {0, 0})   = 1 (in [6], left {0, 0})
read(6, "1\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)
select(16, [6 9], NULL, NULL, {0, 0})   = 1 (in [6], left {0, 0})
read(6, "2\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)
select(16, [6 9], NULL, NULL, {0, 0})   = 1 (in [6], left {0, 0})
read(6, "3\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)
--- SIGCHLD (Child exited) @ 0 (0) ---
select(16, [6 9], NULL, NULL, {0, 0})   = 2 (in [6 9], left {0, 0})
read(6, "4\n", 4096)                    = 2
read(9, "0\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)
--- SIGCHLD (Child exited) @ 0 (0) ---
select(16, [6 9], NULL, NULL, {0, 0})   = 2 (in [6 9], left {0, 0})
read(6, "foo\nbar\n", 4096)             = 8                         <--- !!
read(9, "0\n", 4096)                    = 2
select(16, [6 9], NULL, NULL, {0, 0})   = 0 (Timeout)

##</code><code>##

        pipe $reader, $writer;
        $reader->blocking(0);
        ...

            if (fileno($fh) == fileno($reader)) {
                read $reader, my $line, 1e5;
                $output .= $line;
            }
            ...