Can I legally use the name of a fictional podcast as a title for a real podcast?

Trademarks operate as source identifiers. If you use a title that makes people think your podcast is originating from a different source and the owner of the product wants to come after you, you could have an issue.  Put another way, if you start a Fun with Flags podcast and people visit thinking it might be provided by the producers of Big Bang Theory, the producers of Big Bang Theory could shut you down.  The Wizard of Oz owners are a great example of this. Anyone who ever tries to adopt a trademark using a Wizard of Oz reference gets a cease and desist letter and will eventually get sued if they persist.  They guard the elements of that movie zealously.