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open FILE, "|du -sk|" or die "$!";
print FILE `ls`;
print while(<FILE>);

The open2() function runs the given $cmd and connects $rdrfh
for reading and $wtrfh for writing.  It's what you think
...
   $pid = open(HANDLE, "|cmd args|");

The write filehandle will have autoflush turned on.

use IPC::Open2;

...
           # or without using the shell
         $pid = open2($rdrfh, $wtrfh, 'some', 'cmd', 'and', 'args');