for (@filenames) {
    s{           # substitute 
      CSS        # the string CSS 
      (          # and a capture consisting in
        \.       # a literal dot,
        [^\.]+   # any number of non-dots,
        \.       # another literal dot,
        ErrorLog # and the constant ErrorLog
      )          # (end of capture)
      $          # anchored at the end of the string
    }
    {$1}x;       # against the capture above
}