Took me about an hour to find code that has different output with and without that line... But it was fun;), so I won't spoil...

$foo='hello'; for(1..5){ $foo=~/(?=(.))/g; pos($foo)=pos($foo); print $1 } [download] "The additional state of being matched with zero-length is associated with the matched string, and is reset by each assignment to pos()."