Some perl stuff added before the code in wp-config.php

For my great surprise while reading the error log about DB deadlock I found out the wp-config.php is silently modified with the following:

[ Malware script deleted ]

and the normal wp-config.php code goes here…

So what the hell is this and how come it got there without changing the file modification date!?!?! Did not know it is even possible – the file date shows some old date. Running W3 Total Cache, All In One WP Security, Jetpack, Block bad queries & bunchado more. Any idea?

Thanks!
-Paul