This is a substitution expression that changes forward slash to a backward slashes. The windows command line needs to have backward slashes while Unix uses forward slashes. With Perl you can use forward slash on both types of systems provided that this is in a Perl function call not a shell cmd to the native OS. I always use forward slash in Perl code. The backward slash has a purpose as an escape character, hence the need for this backslash backslash to mean one backslash. That is not an issue for the forward slash and resulting regular expressions are easier to understand.