Where is this buffer??

It's part of the system file handle.

Doesnt it count as memory?

It holds data, so it's some form of memory by definition.

But it's a rather small (64k?), fixed-size buffer.

So according to what you said the output is stored in this buffer. Backticks empty them into a scalar all at once.

No. That would require the output of the child to fit in the buffer, but that's impossible since it's a fixed-size buffer. Backticks repeatedly and continually empty the pipe into a scalar. Backticks is more or less equivalent to 

my $pid = open(my $fh, '-|', $cmd);
my $scalar = '';
1 while sysread($fh, $scalar, BLK_SIZE, length($scalar));
waitpid($pid, 0);
return $scalar;

[download]

The scalar keep growing and growing.

Update: Added the first two answers.

In reply to Re^3: Executing Commands with "open" by ikegami
in thread Executing Commands with "open" by abhijithtk

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