At the Perl level, syswrite reports that all the data has been written. At the OS level, a process monitoring tool such as truss shows the write call also reporting that all the data has been written.

And BTW, the dropped data is not the chunk tail but the head!

update: this is the interaction between both processes at the OS level:

27168:  write(4, "\0\0\00501\0\0\003", 9) = 9
27170:  read(7, "\0\0\00501\0\0\003", 16384) = 9
27170:  write(8, "\0\0\00502\0\0\003", 9) = 9
27168:  read(4, "\0\0\00502\0\0\003", 16384) = 9
27168:  write(4, "\0\0\0" 03\0\0\0\0\0\0\0\r/ t m ".., 38) = 38
27170:  read(7, "\0\0\0" 03\0\0\0\0\0\0\0\r/ t m ".., 16384) = 38
27170:  write(8, "\0\0\0\rf \0\0\0\0\0\0\004\0\0\0".., 17) = 17
27168:  read(4, "\0\0\0\rf \0\0\0\0\0\0\004\0\0\0".., 16384) = 17
27168:  write(4, "\0\0\015\n\0\0\001\0\0\004\0\0\0".., 25) = 25
27170:  read(7, "\0\0\015\n\0\0\001\0\0\004\0\0\0".., 16384) = 25
27170:  write(8, "\0\0\018e \0\0\001\0\0\0\0\0\0\0".., 28) = 28
27168:  read(4, "\0\0\018e \0\0\001\0\0\0\0\0\0\0".., 16384) = 28
27168:  write(4, "\0\0@ 1906\0\0\002\0\0\004\0\0\0".., 16413) = 16413
27170:  read(7, "a l / l i b \nL I B C   =   \nL ".., 16384) = 1024
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Note how reads and writes for both processes interleave until the last one where only 1024 bytes are read by the child and that those bytes do not correspond to the head of the last write operation performed on the parent!