It's probably interpreting your hyphen in the character class as indicating a range, so you probably need to escape it:
list($title1, $title2) = split('[/\-._]', $title);
You might get better performance with preg_split, though that's probably negligible in this small, simple example:
list($title1, $title2) = preg_split('#[/\-._]#', $title);