open FILE, "|du -sk|" or die "$!";
print FILE `ls`;
print while(<FILE>);
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Edit: This is false... :(
As noted in the replies, IPC::Open2 was the correct solution. From perldoc IPC::Open2:

The open2() function runs the given $cmd and connects $rdrfh
for reading and $wtrfh for writing.  It's what you think
should work when you try

   $pid = open(HANDLE, "|cmd args|");

The write filehandle will have autoflush turned on.
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That was what confused my. I'm sorry for the error.
This is the syntax for Open2:

use IPC::Open2;

         $pid = open2(\*RDRFH, \*WTRFH, 'some cmd and args');
           # or without using the shell
         $pid = open2(\*RDRFH, \*WTRFH, 'some', 'cmd', 'and', 'args');

         # or with handle autovivification
         my($rdrfh, $wtrfh);
         $pid = open2($rdrfh, $wtrfh, 'some cmd and args');
           # or without using the shell
         $pid = open2($rdrfh, $wtrfh, 'some', 'cmd', 'and', 'args');
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Open3 gives you also STDERR.