this works too.

#!/usr/bin/perl -w

use strict;
use IPC::Open2;

$|=1;

open2( my $read, my $write, "top -b" );
while( <$read> )   {
   print if /CPU states/ ;
}
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also if you try those piping methods again and wait for a really really long time (like 3 or 4 minutes) you will eventually get output. not sure what the deal is exactly there. guess the buffer isn't flushing? often? or something? for example, try top -b | grep "CPU states | less and wait for a while. it will eventually come up with some output.