There's a couple of problems with your code (buffering, sending 1025 or 1026 bytes at a time, -n absorbs some of the bytes).

On this Debian system:

$ perl -e'$|=1; { print("x"x1024); warn(++$i); redo }' | perl -e sleep
...
60 at -e line 1.
61 at -e line 1.
62 at -e line 1.
63 at -e line 1.
64 at -e line 1.
[blocks]
^C
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