Why can't I use periods when searching from the location bar?

Midori tries to make a smart guess on whether you are searching for something or typing a valid address by hand. Yet there are ambiguous cases so it fails.

Try using 'g' in front of the search terms, like 'g Ms.Marple'. The 'g' is a token that tells the browser you want to use Google. See Tools > Manage Search Engines or right-click the location bar and select the menu item, to look at search engines and tokens.